diff --git a/web/base_tree_viewer.js b/web/base_tree_viewer.js
new file mode 100644
index 000000000..579cd0147
--- /dev/null
+++ b/web/base_tree_viewer.js
@@ -0,0 +1,111 @@
+/* Copyright 2020 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { removeNullCharacters } from "pdfjs-lib";
+
+class BaseTreeViewer {
+  constructor(options) {
+    if (this.constructor === BaseTreeViewer) {
+      throw new Error("Cannot initialize BaseTreeViewer.");
+    }
+    this.container = options.container;
+    this.eventBus = options.eventBus;
+
+    this.reset();
+  }
+
+  reset() {
+    this._lastToggleIsShow = true;
+
+    // Remove the tree from the DOM.
+    this.container.textContent = "";
+    // Ensure that the left (right in RTL locales) margin is always reset,
+    // to prevent incorrect tree alignment if a new document is opened.
+    this.container.classList.remove("treeWithDeepNesting");
+  }
+
+  /**
+   * @private
+   */
+  _dispatchEvent(count) {
+    throw new Error("Not implemented: _dispatchEvent");
+  }
+
+  /**
+   * @private
+   */
+  _bindLink(element, params) {
+    throw new Error("Not implemented: _bindLink");
+  }
+
+  /**
+   * @private
+   */
+  _normalizeTextContent(str) {
+    return removeNullCharacters(str) || /* en dash = */ "\u2013";
+  }
+
+  /**
+   * Prepend a button before a tree item which allows the user to collapse or
+   * expand all tree items at that level; see `_toggleTreeItem`.
+   * @private
+   */
+  _addToggleButton(div, hidden = false) {
+    const toggler = document.createElement("div");
+    toggler.className = "treeItemToggler";
+    if (hidden) {
+      toggler.classList.add("treeItemsHidden");
+    }
+    toggler.onclick = evt => {
+      evt.stopPropagation();
+      toggler.classList.toggle("treeItemsHidden");
+
+      if (evt.shiftKey) {
+        const shouldShowAll = !toggler.classList.contains("treeItemsHidden");
+        this._toggleTreeItem(div, shouldShowAll);
+      }
+    };
+    div.insertBefore(toggler, div.firstChild);
+  }
+
+  /**
+   * Collapse or expand the subtree of a tree item.
+   *
+   * @param {Element} root - the root of the item (sub)tree.
+   * @param {boolean} show - whether to show the item (sub)tree. If false,
+   *   the item subtree rooted at `root` will be collapsed.
+   * @private
+   */
+  _toggleTreeItem(root, show = false) {
+    this._lastToggleIsShow = show;
+    for (const toggler of root.querySelectorAll(".treeItemToggler")) {
+      toggler.classList.toggle("treeItemsHidden", !show);
+    }
+  }
+
+  /**
+   * Collapse or expand all subtrees of the `container`.
+   * @private
+   */
+  _toggleAllTreeItems() {
+    this._toggleTreeItem(this.container, !this._lastToggleIsShow);
+  }
+
+  render(params) {
+    throw new Error("Not implemented: render");
+  }
+}
+
+export { BaseTreeViewer };
diff --git a/web/pdf_attachment_viewer.js b/web/pdf_attachment_viewer.js
index 0ba8715ff..b9bd851bf 100644
--- a/web/pdf_attachment_viewer.js
+++ b/web/pdf_attachment_viewer.js
@@ -13,11 +13,8 @@
  * limitations under the License.
  */
 
-import {
-  createPromiseCapability,
-  getFilenameFromUrl,
-  removeNullCharacters,
-} from "pdfjs-lib";
+import { createPromiseCapability, getFilenameFromUrl } from "pdfjs-lib";
+import { BaseTreeViewer } from "./base_tree_viewer.js";
 
 /**
  * @typedef {Object} PDFAttachmentViewerOptions
@@ -31,16 +28,13 @@ import {
  * @property {Object|null} attachments - A lookup table of attachment objects.
  */
 
-class PDFAttachmentViewer {
+class PDFAttachmentViewer extends BaseTreeViewer {
   /**
    * @param {PDFAttachmentViewerOptions} options
    */
-  constructor({ container, eventBus, downloadManager }) {
-    this.container = container;
-    this.eventBus = eventBus;
-    this.downloadManager = downloadManager;
-
-    this.reset();
+  constructor(options) {
+    super(options);
+    this.downloadManager = options.downloadManager;
 
     this.eventBus._on(
       "fileattachmentannotation",
@@ -49,10 +43,8 @@ class PDFAttachmentViewer {
   }
 
   reset(keepRenderedCapability = false) {
-    this.attachments = null;
-
-    // Remove the attachments from the DOM.
-    this.container.textContent = "";
+    super.reset();
+    this._attachments = null;
 
     if (!keepRenderedCapability) {
       // The only situation in which the `_renderedCapability` should *not* be
@@ -100,9 +92,9 @@ class PDFAttachmentViewer {
    * NOTE: Should only be used when `URL.createObjectURL` is natively supported.
    * @private
    */
-  _bindPdfLink(button, content, filename) {
+  _bindPdfLink(element, { content, filename }) {
     let blobUrl;
-    button.onclick = () => {
+    element.onclick = () => {
       if (!blobUrl) {
         blobUrl = URL.createObjectURL(
           new Blob([content], { type: "application/pdf" })
@@ -141,8 +133,8 @@ class PDFAttachmentViewer {
   /**
    * @private
    */
-  _bindLink(button, content, filename) {
-    button.onclick = () => {
+  _bindLink(element, { content, filename }) {
+    element.onclick = () => {
       this.downloadManager.downloadData(content, filename, "");
       return false;
     };
@@ -152,42 +144,45 @@ class PDFAttachmentViewer {
    * @param {PDFAttachmentViewerRenderParameters} params
    */
   render({ attachments, keepRenderedCapability = false }) {
-    if (this.attachments) {
-      this.reset(keepRenderedCapability === true);
+    if (this._attachments) {
+      this.reset(keepRenderedCapability);
     }
-    this.attachments = attachments || null;
+    this._attachments = attachments || null;
 
     if (!attachments) {
       this._dispatchEvent(/* attachmentsCount = */ 0);
       return;
     }
-
     const names = Object.keys(attachments).sort(function (a, b) {
       return a.toLowerCase().localeCompare(b.toLowerCase());
     });
-    const attachmentsCount = names.length;
 
     const fragment = document.createDocumentFragment();
-    for (let i = 0; i < attachmentsCount; i++) {
-      const item = attachments[names[i]];
-      const filename = removeNullCharacters(getFilenameFromUrl(item.filename));
+    let attachmentsCount = 0;
+    for (const name of names) {
+      const item = attachments[name];
+      const filename = getFilenameFromUrl(item.filename);
 
       const div = document.createElement("div");
-      div.className = "attachmentsItem";
-      const button = document.createElement("button");
-      button.textContent = filename;
+      div.className = "treeItem";
+
+      const element = document.createElement("a");
       if (
         /\.pdf$/i.test(filename) &&
         !this.downloadManager.disableCreateObjectURL
       ) {
-        this._bindPdfLink(button, item.content, filename);
+        this._bindPdfLink(element, { content: item.content, filename });
       } else {
-        this._bindLink(button, item.content, filename);
+        this._bindLink(element, { content: item.content, filename });
       }
+      element.textContent = this._normalizeTextContent(filename);
+
+      div.appendChild(element);
 
-      div.appendChild(button);
       fragment.appendChild(div);
+      attachmentsCount++;
     }
+
     this.container.appendChild(fragment);
 
     this._dispatchEvent(attachmentsCount);
@@ -204,7 +199,7 @@ class PDFAttachmentViewer {
       if (renderedPromise !== this._renderedCapability.promise) {
         return; // The FileAttachment annotation belongs to a previous document.
       }
-      let attachments = this.attachments;
+      let attachments = this._attachments;
 
       if (!attachments) {
         attachments = Object.create(null);
diff --git a/web/pdf_outline_viewer.js b/web/pdf_outline_viewer.js
index 672e3b76c..bb489b7a2 100644
--- a/web/pdf_outline_viewer.js
+++ b/web/pdf_outline_viewer.js
@@ -13,9 +13,8 @@
  * limitations under the License.
  */
 
-import { addLinkAttributes, LinkTarget, removeNullCharacters } from "pdfjs-lib";
-
-const DEFAULT_TITLE = "\u2013";
+import { addLinkAttributes, LinkTarget } from "pdfjs-lib";
+import { BaseTreeViewer } from "./base_tree_viewer.js";
 
 /**
  * @typedef {Object} PDFOutlineViewerOptions
@@ -29,30 +28,20 @@ const DEFAULT_TITLE = "\u2013";
  * @property {Array|null} outline - An array of outline objects.
  */
 
-class PDFOutlineViewer {
+class PDFOutlineViewer extends BaseTreeViewer {
   /**
    * @param {PDFOutlineViewerOptions} options
    */
-  constructor({ container, linkService, eventBus }) {
-    this.container = container;
-    this.linkService = linkService;
-    this.eventBus = eventBus;
+  constructor(options) {
+    super(options);
+    this.linkService = options.linkService;
 
-    this.reset();
-
-    eventBus._on("toggleoutlinetree", this.toggleOutlineTree.bind(this));
+    this.eventBus._on("toggleoutlinetree", this._toggleAllTreeItems.bind(this));
   }
 
   reset() {
-    this.outline = null;
-    this.lastToggleIsShow = true;
-
-    // Remove the outline from the DOM.
-    this.container.textContent = "";
-
-    // Ensure that the left (right in RTL locales) margin is always reset,
-    // to prevent incorrect outline alignment if a new document is opened.
-    this.container.classList.remove("outlineWithDeepNesting");
+    super.reset();
+    this._outline = null;
   }
 
   /**
@@ -103,84 +92,51 @@ class PDFOutlineViewer {
   }
 
   /**
-   * Prepend a button before an outline item which allows the user to toggle
-   * the visibility of all outline items at that level.
-   *
    * @private
    */
   _addToggleButton(div, { count, items }) {
-    const toggler = document.createElement("div");
-    toggler.className = "outlineItemToggler";
-    if (count < 0 && Math.abs(count) === items.length) {
-      toggler.classList.add("outlineItemsHidden");
-    }
-    toggler.onclick = evt => {
-      evt.stopPropagation();
-      toggler.classList.toggle("outlineItemsHidden");
-
-      if (evt.shiftKey) {
-        const shouldShowAll = !toggler.classList.contains("outlineItemsHidden");
-        this._toggleOutlineItem(div, shouldShowAll);
-      }
-    };
-    div.insertBefore(toggler, div.firstChild);
+    const hidden = count < 0 && Math.abs(count) === items.length;
+    super._addToggleButton(div, hidden);
   }
 
   /**
-   * Toggle the visibility of the subtree of an outline item.
-   *
-   * @param {Element} root - the root of the outline (sub)tree.
-   * @param {boolean} show - whether to show the outline (sub)tree. If false,
-   *   the outline subtree rooted at |root| will be collapsed.
-   *
    * @private
    */
-  _toggleOutlineItem(root, show = false) {
-    this.lastToggleIsShow = show;
-    for (const toggler of root.querySelectorAll(".outlineItemToggler")) {
-      toggler.classList.toggle("outlineItemsHidden", !show);
-    }
-  }
-
-  /**
-   * Collapse or expand all subtrees of the outline.
-   */
-  toggleOutlineTree() {
-    if (!this.outline) {
+  _toggleAllTreeItems() {
+    if (!this._outline) {
       return;
     }
-    this._toggleOutlineItem(this.container, !this.lastToggleIsShow);
+    super._toggleAllTreeItems();
   }
 
   /**
    * @param {PDFOutlineViewerRenderParameters} params
    */
   render({ outline }) {
-    let outlineCount = 0;
-
-    if (this.outline) {
+    if (this._outline) {
       this.reset();
     }
-    this.outline = outline || null;
+    this._outline = outline || null;
 
     if (!outline) {
-      this._dispatchEvent(outlineCount);
+      this._dispatchEvent(/* outlineCount = */ 0);
       return;
     }
 
     const fragment = document.createDocumentFragment();
-    const queue = [{ parent: fragment, items: this.outline }];
-    let hasAnyNesting = false;
+    const queue = [{ parent: fragment, items: outline }];
+    let outlineCount = 0,
+      hasAnyNesting = false;
     while (queue.length > 0) {
       const levelData = queue.shift();
       for (const item of levelData.items) {
         const div = document.createElement("div");
-        div.className = "outlineItem";
+        div.className = "treeItem";
 
         const element = document.createElement("a");
         this._bindLink(element, item);
         this._setStyles(element, item);
-        element.textContent = removeNullCharacters(item.title) || DEFAULT_TITLE;
+        element.textContent = this._normalizeTextContent(item.title);
 
         div.appendChild(element);
 
@@ -189,8 +145,9 @@ class PDFOutlineViewer {
           this._addToggleButton(div, item);
 
           const itemsDiv = document.createElement("div");
-          itemsDiv.className = "outlineItems";
+          itemsDiv.className = "treeItems";
           div.appendChild(itemsDiv);
+
           queue.push({ parent: itemsDiv, items: item.items });
         }
 
@@ -199,10 +156,10 @@ class PDFOutlineViewer {
       }
     }
     if (hasAnyNesting) {
-      this.container.classList.add("outlineWithDeepNesting");
+      this.container.classList.add("treeWithDeepNesting");
 
-      this.lastToggleIsShow =
-        fragment.querySelectorAll(".outlineItemsHidden").length === 0;
+      this._lastToggleIsShow =
+        fragment.querySelectorAll(".treeItemsHidden").length === 0;
     }
 
     this.container.appendChild(fragment);
diff --git a/web/viewer.css b/web/viewer.css
index d49e0f6f5..c33e5674b 100644
--- a/web/viewer.css
+++ b/web/viewer.css
@@ -1169,30 +1169,23 @@ a:focus > .thumbnail > .thumbnailSelectionRing,
   width: calc(100% - 8px);
   top: 0;
   bottom: 0;
+  padding: 4px 4px 0;
   overflow: auto;
   -webkit-overflow-scrolling: touch;
   user-select: none;
 }
 
-#outlineView {
-  padding: 4px 4px 0;
-}
-#attachmentsView {
-  padding: 3px 4px 0;
-}
-
-html[dir='ltr'] .outlineWithDeepNesting > .outlineItem,
-html[dir='ltr'] .outlineItem > .outlineItems {
+html[dir='ltr'] .treeWithDeepNesting > .treeItem,
+html[dir='ltr'] .treeItem > .treeItems {
   margin-left: 20px;
 }
 
-html[dir='rtl'] .outlineWithDeepNesting > .outlineItem,
-html[dir='rtl'] .outlineItem > .outlineItems {
+html[dir='rtl'] .treeWithDeepNesting > .treeItem,
+html[dir='rtl'] .treeItem > .treeItems {
   margin-right: 20px;
 }
 
-.outlineItem > a,
-.attachmentsItem > button {
+.treeItem > a {
   text-decoration: none;
   display: inline-block;
   min-width: 95%;
@@ -1206,69 +1199,52 @@ html[dir='rtl'] .outlineItem > .outlineItems {
   line-height: 15px;
   user-select: none;
   white-space: normal;
-}
-
-.attachmentsItem > button {
-  border: 0 none;
-  background: none;
   cursor: pointer;
-  width: 100%;
 }
-
-html[dir='ltr'] .outlineItem > a {
+html[dir='ltr'] .treeItem > a {
   padding: 2px 0 5px 4px;
 }
-html[dir='ltr'] .attachmentsItem > button {
-  padding: 2px 0 3px 7px;
-  text-align: left;
-}
-
-html[dir='rtl'] .outlineItem > a {
+html[dir='rtl'] .treeItem > a {
   padding: 2px 4px 5px 0;
 }
-html[dir='rtl'] .attachmentsItem > button {
-  padding: 2px 7px 3px 0;
-  text-align: right;
-}
 
-.outlineItemToggler {
+.treeItemToggler {
   position: relative;
   height: 0;
   width: 0;
   color: rgba(255, 255, 255, 0.5);
 }
-.outlineItemToggler::before {
+.treeItemToggler::before {
   content: url(images/treeitem-expanded.png);
   display: inline-block;
   position: absolute;
 }
-.outlineItemToggler.outlineItemsHidden::before {
+.treeItemToggler.treeItemsHidden::before {
   content: url(images/treeitem-collapsed.png);
 }
-html[dir='rtl'] .outlineItemToggler.outlineItemsHidden::before {
+html[dir='rtl'] .treeItemToggler.treeItemsHidden::before {
   transform: scaleX(-1);
 }
-.outlineItemToggler.outlineItemsHidden ~ .outlineItems {
+.treeItemToggler.treeItemsHidden ~ .treeItems {
   display: none;
 }
-html[dir='ltr'] .outlineItemToggler {
+html[dir='ltr'] .treeItemToggler {
   float: left;
 }
-html[dir='rtl'] .outlineItemToggler {
+html[dir='rtl'] .treeItemToggler {
   float: right;
 }
-html[dir='ltr'] .outlineItemToggler::before {
+html[dir='ltr'] .treeItemToggler::before {
   right: 4px;
 }
-html[dir='rtl'] .outlineItemToggler::before {
+html[dir='rtl'] .treeItemToggler::before {
   left: 4px;
 }
 
-.outlineItemToggler:hover,
-.outlineItemToggler:hover + a,
-.outlineItemToggler:hover ~ .outlineItems,
-.outlineItem > a:hover,
-.attachmentsItem > button:hover {
+.treeItemToggler:hover,
+.treeItemToggler:hover + a,
+.treeItemToggler:hover ~ .treeItems,
+.treeItem > a:hover {
   background-color: rgba(255, 255, 255, 0.02);
   background-image: linear-gradient(rgba(255, 255, 255, 0.05), rgba(255, 255, 255, 0));
   background-clip: padding-box;
@@ -1279,7 +1255,7 @@ html[dir='rtl'] .outlineItemToggler::before {
   color: rgba(255, 255, 255, 0.9);
 }
 
-.outlineItem.selected {
+.treeItem.selected {
   background-color: rgba(255, 255, 255, 0.08);
   background-image: linear-gradient(rgba(255, 255, 255, 0.05), rgba(255, 255, 255, 0));
   background-clip: padding-box;
@@ -1745,21 +1721,21 @@ html[dir='rtl'] #documentPropertiesOverlay .row > * {
     content: url(images/secondaryToolbarButton-documentProperties@2x.png);
   }
 
-  .outlineItemToggler::before {
+  .treeItemToggler::before {
     transform: scale(0.5);
     top: -1px;
     content: url(images/treeitem-expanded@2x.png);
   }
-  .outlineItemToggler.outlineItemsHidden::before {
+  .treeItemToggler.treeItemsHidden::before {
     content: url(images/treeitem-collapsed@2x.png);
   }
-  html[dir='rtl'] .outlineItemToggler.outlineItemsHidden::before {
+  html[dir='rtl'] .treeItemToggler.treeItemsHidden::before {
     transform: scale(-0.5, 0.5);
   }
-  html[dir='ltr'] .outlineItemToggler::before {
+  html[dir='ltr'] .treeItemToggler::before {
     right: 0;
   }
-  html[dir='rtl'] .outlineItemToggler::before {
+  html[dir='rtl'] .treeItemToggler::before {
     left: 0;
   }
 }