Set tab order when document is tagged

lib/mixins/markings.js

@@ -138,6 +138,10 @@ export default {
     return this._root.data.MarkInfo;
   },
 
+  hasMarkInfoDictionary() {
+    return !!this._root.data.MarkInfo;
+  },
+
   getStructTreeRoot() {
     if (!this._root.data.StructTreeRoot) {
       this._root.data.StructTreeRoot = this.ref({

lib/page.js

@@ -155,7 +155,15 @@ class PDFPage {
     return this.content.write(chunk);
   }
 
+  // Set tab order if document is tagged for accessibility.
+  _setTabOrder() {
+    if (!this.dictionary.Tabs && this.document.hasMarkInfoDictionary()) {
+      this.dictionary.data.Tabs = 'S';
+    }
+  }
+
   end() {
+    this._setTabOrder();
     this.dictionary.end();
     this.resources.end();
     return this.content.end();

tests/unit/markings.spec.js

@@ -623,6 +623,47 @@ EMC
         `(My Title)`,
         `endobj`
       ]);
+      expect(docData).toContainChunk([
+        `10 0 obj`,
+        /\/Tabs \/S/,
+        `endobj`
+      ]);
+    });
+  });
+
+  describe('untagged document', () => {
+    test('taborder not set for unmarked content', () => {
+      document = new PDFDocument({
+        info: {
+          CreationDate: new Date(Date.UTC(2018, 1, 1)),
+          Title: "My Title"
+        },
+        displayTitle: true,
+        compress: false,
+        pdfVersion: '1.5',
+        tagged: false,
+        lang: 'en-AU'
+      });
+
+      const docData = logData(document);
+
+      document.end();
+
+      expect(docData).toContainChunk([
+        `3 0 obj`,
+        /\/Lang \(en-AU\)/,
+        `endobj`
+      ]);
+      expect(docData).not.toContainChunk([
+        `3 0 obj`,
+        /\/MarkInfo 5 0 R/,
+        `endobj`
+      ]);
+      expect(docData).not.toContainChunk([
+        `10 0 obj`,
+        /\/Tabs \/S/,
+        `endobj`
+      ]);
     });
   });