The scroll-and-hunt problem with long documents
Open any long PDF without bookmarks — a 60-page report, a manual, a compiled set of documents — and there's exactly one way to find a specific section: scroll and squint, or use text search if you remember the right words to search for. Compare that to opening a PDF that has a proper sidebar table of contents, where a single click jumps straight to the chapter you need. That sidebar isn't magic; it's a feature called bookmarks (technically 'outlines' in the PDF specification), and plenty of PDFs simply don't have them, especially ones assembled by merging several documents together or exported from software that doesn't add them automatically.
This matters more than it might seem for anything people will actually reference repeatedly rather than read once start to finish — a policy document, a reference manual, a compiled report with distinct sections. Without navigation, every single lookup costs someone real time.
What a bookmark actually is under the hood
A PDF bookmark isn't a visual trick or an approximation — it's a real structural element defined by the PDF specification: a linked list of outline entries, each one carrying a title and a destination pointing to a specific page. Because this is standard, spec-compliant structure, bookmarks you add show up correctly in the navigation panel of any PDF reader — Adobe Acrobat, a browser's built-in viewer, a mobile PDF app — exactly the same way bookmarks from any professional PDF-authoring software would.
Building this list is straightforward: give each entry a title (whatever text should appear in the navigation panel) and the page number it should jump to, and add as many entries as your document's structure needs. Review the full list before saving, since fixing the order or wording after the fact just means editing the same list again, not starting over.
Structuring bookmarks people will actually use
Match your bookmark titles to how someone would actually think about the section, not necessarily the document's literal heading text — 'Refund Policy' is more useful in a navigation panel than 'Section 4.2,' even if the page itself is headed 'Section 4.2.' The point of a bookmark is to be found by someone scanning a list of options, not to mirror the document's internal numbering scheme.
For a document with clearly distinct major sections — chapters, parts, appendices — one bookmark per major section is usually enough; you don't need one for every subsection or every page, since a bookmark list that's too long defeats the purpose of quick navigation just as much as having none at all.
What this tool doesn't do
This produces a flat list of bookmarks, not a nested, hierarchical outline (a top-level chapter bookmark with indented sub-section bookmarks underneath it). For most documents a flat list of major sections covers the actual need, but if you're working with something that genuinely requires multi-level navigation — a large technical manual with deep sub-sections, for instance — a flat list is a simplification, not a full substitute for proper nested outlines.
It's also worth knowing that adding a new bookmark outline may replace an existing one if the document already had bookmarks, since a PDF holds a single outline structure rather than multiple independent ones — check whether your source document already has useful bookmarks before overwriting them.