diff --git a/l10n/en-US/viewer.properties b/l10n/en-US/viewer.properties
index f75ceabbb..731a1f228 100644
--- a/l10n/en-US/viewer.properties
+++ b/l10n/en-US/viewer.properties
@@ -99,8 +99,8 @@ print_progress_close=Cancel
 # tooltips)
 toggle_sidebar.title=Toggle Sidebar
 toggle_sidebar_label=Toggle Sidebar
-outline.title=Show Document Outline
-outline_label=Document Outline
+document_outline.title=Show Document Outline (double-click to expand/collapse all items)
+document_outline_label=Document Outline
 attachments.title=Show Attachments
 attachments_label=Attachments
 thumbs.title=Show Thumbnails
diff --git a/web/viewer.html b/web/viewer.html
index 676b56cda..566770ae4 100644
--- a/web/viewer.html
+++ b/web/viewer.html
@@ -81,8 +81,8 @@ See https://github.com/adobe-type-tools/cmap-resources
             <button id="viewThumbnail" class="toolbarButton group toggled" title="Show Thumbnails" tabindex="2" data-l10n-id="thumbs">
                <span data-l10n-id="thumbs_label">Thumbnails</span>
             </button>
-            <button id="viewOutline" class="toolbarButton group" title="Show Document Outline" tabindex="3" data-l10n-id="outline">
-               <span data-l10n-id="outline_label">Document Outline</span>
+            <button id="viewOutline" class="toolbarButton group" title="Show Document Outline (double-click to expand/collapse all items)" tabindex="3" data-l10n-id="document_outline">
+               <span data-l10n-id="document_outline_label">Document Outline</span>
             </button>
             <button id="viewAttachments" class="toolbarButton group" title="Show Attachments" tabindex="4" data-l10n-id="attachments">
                <span data-l10n-id="attachments_label">Attachments</span>