We’re not talking about strips of cardboard or dog-eared pages. When present, PDF bookmarks appear to the left of the page (the F4 key toggles the bookmarks “panel” on and off). In the most common application, bookmarks allow users to instantly skip to other locations in the document. That’s VERY handy, especially for longer documents.
Bookmarks are most typically employed as a clickable table of contents. Unlike a table of contents, which exists as a page within the document, bookmarks remain on screen as long as you need them. Transporting the user to any desired heading within one or more PDF documents, bookmarks may also trigger almost any other action available in PDF, such as a link to another website, JavaScript code, a “Print” command and so on.
Often, bookmarks are the very first interactive feature users want to see in a PDF file. The all-too common lack of bookmarks is a simple explanation for most of usability guru Jakob Nielsen’s famous gripes about PDF. Without bookmarks, finding a specific subheading in a PDF file a mere 10 pages in length can be a minor exercise in frustration, even for advanced users. If the PDF is very long, or serves a reference document where swift access to any subsection is at a premium, frustration can quickly grow. The real value of bookmarks for everyday document navigation is illustrated in Bookmarks: Why and How.
In addition to document-navigation needs, bookmarks deliver other advanced functionality through a smooth, consistent, omnipresent and therefore familiar interface. Beyond hierarchical tables of contents, bookmarks can easily unify navigation among multiple PDFs into a single, simple interface, provide a index of tables, figures, part numbers and so on. Bookmarks have even been used as triggers for audio commentary. Some authors dispense with the toolbar altogether, opting to offer bookmarks as the ONLY interface for the functions they consider desirable. The possibilities are almost endless.
https://talkingpdf.org/review-pdf-bookmarks-surveying-the-options/