Add a default DA for textfield to avoid issues when printing or saving

* it aims to fix issue #12750

src/core/annotation.js

@@ -994,10 +994,13 @@ class WidgetAnnotation extends Annotation {
     data.defaultFieldValue = this._decodeFormValue(defaultFieldValue);
 
     data.alternativeText = stringToPDFString(dict.get("TU") || "");
-    data.defaultAppearance =
+    const defaultAppearance =
       getInheritableProperty({ dict, key: "DA" }) ||
       params.acroForm.get("DA") ||
       "";
+    data.defaultAppearance = isString(defaultAppearance)
+      ? defaultAppearance
+      : "";
     const fieldType = getInheritableProperty({ dict, key: "FT" });
     data.fieldType = isName(fieldType) ? fieldType.name : null;
 
@@ -1282,6 +1285,15 @@ class WidgetAnnotation extends Annotation {
     const totalHeight = this.data.rect[3] - this.data.rect[1];
     const totalWidth = this.data.rect[2] - this.data.rect[0];
 
+    if (!this.data.defaultAppearance) {
+      // The DA is required and must be a string.
+      // If there is no font named Helvetica in the resource dictionary,
+      // the evaluator will fall back to a default font.
+      // Doing so prevents exceptions and allows saving/printing
+      // the file as expected.
+      this.data.defaultAppearance = "/Helvetica 0 Tf 0 g";
+    }
+
     const fontInfo = await this._getFontData(evaluator, task);
     const [font, fontName] = fontInfo;
     const fontSize = this._computeFontSize(...fontInfo, totalHeight);

test/pdfs/.gitignore

@@ -48,6 +48,7 @@
 !issue6931_reduced.pdf
 !issue7580.pdf
 !issue7598.pdf
+!issue12750.pdf
 !issue7665.pdf
 !issue7696.pdf
 !issue7835.pdf

test/test_manifest.json

@@ -2779,6 +2779,18 @@
       "link": true,
       "type": "eq"
     },
+    {  "id": "issue12750",
+       "file": "pdfs/issue12750.pdf",
+       "md5": "47a39ef5d6ae2e4f52887c477cfdfc9e",
+       "rounds": 1,
+       "type": "eq",
+       "print": true,
+       "annotationStorage": {
+         "12R": {
+           "value": "foobar"
+         }
+       }
+    },
     {  "id": "preistabelle-text",
        "file": "pdfs/preistabelle.pdf",
        "md5": "d2f0b2086160d4f3d325c79a5dc1fb4d",