If a document has 21 or more pages and does not have bookmark, this check will be marked as a fail in the Adobe Accessibility Full Check. Bookmarks allow users to quickly located content throughout a document. Though the check may pass if your document has less than 21 pages, bookmarks are still strongly encouraged because they are helpful for all users.
If the PDF is tagged, bookmarks can be automatically added. In the Accessibility Full Check panel, right-click the failed check and select "Fix." In the "Structure Elements" dialog box, select the element(s) that should be used as bookmarks (headings are typically a good indicator of where a bookmark should go), and click "OK" to add the bookmarks. Review the bookmarks in the left-hand side panel to ensure that they are correct.
The "Structure Elements" dialog box can also be accessed by clicking the Options menu on the Bookmarks tab and selecting "New Bookmarks from Structure."
Bookmarks can be added with the Edit PDF tool, too. With the Edit PDF tool open, press the Control key plus the letter B on the keyboard (or Command and B for Mac users) on the keyboard to add a bookmark where your cursor is located in the document.
Another way to manually add bookmarks is to use the Accessibility tool, find the Bookmark icon on the left-hand side panel and select the New Bookmark icon. The bookmark will be added based on the location of the cursor in the document. Enter a name for the bookmark and click out of the bookmark to save it. To edit a bookmark, click the bookmark name or right-click the bookmark that needs to be changed and select "Rename."