PDF Documents with multiple pages must have bookmarks
The bookmarks audit is based on the Adobe accessibility best practice rules.
This check fails when the document has 21 or more pages, but doesn't have bookmarks that parallel the document structure.
How to fix (Adobe Pro)?
To add bookmarks to the document, select Bookmarks on the Accessibility Checker panel, and choose Fix from the Options menu.
In the Structure Elements dialog box, select the elements that you want to use as bookmarks, and click OK. (You can also access the Structure Elements dialog box by clicking the Options menu on the Bookmark tab and selecting the New Bookmarks From Structure command.)