Why did my PDF's bookmarks disappear after merging?

A PDF's clickable bookmarks (its outline tree) are stored separately from the pages themselves, in the document catalog. Merge tools that work by copying pages from one PDF into a new document — which is how most lightweight and browser-based mergers work — carry the page content over but don't rebuild that separate outline tree, so the bookmarks are silently dropped.

Is this a bug in the merge tool?

It's a missing feature, not a bug that corrupts your file. The pages, their content, and their internal page-to-page links are intact. Rebuilding a bookmark tree across merged documents is meaningfully more work than copying pages, so many merge tools, including page-copy-based ones, don't implement it.

How do I keep bookmarks when combining PDFs?

If the bookmarks matter, use desktop software that explicitly advertises bookmark or outline preservation during merge (most full PDF editors do this), or manually rebuild a short bookmark list afterward in a tool that supports adding outline entries. There isn't a way to make a page-copy-based merge carry outlines over without that dedicated support.

Why did merging my PDFs lose the bookmarks and table of contents?

ClientPDF Guide · a 2-minute read

You merged a few PDFs — maybe a report with a clickable table of contents, or a book with chapter bookmarks in the sidebar — and the merged file opens fine, all the pages are there, but the bookmark panel is empty. Nothing crashed, no pages are missing. This is a specific, well-understood gap in how most merge tools work, not damage to your file.

Bookmarks live outside the pages, not inside them

A PDF's clickable bookmark list (technically its outline tree) isn't stored as part of any individual page. It's a separate structure hanging off the document's catalog — a tree of named entries, each pointing at a page and a position on it. The pages themselves know nothing about it.

Most lightweight merge tools, including this one, work by copying page objects from each source file into a brand-new PDF document. That's enough to carry over everything visibly printed on the page — text, images, internal page-to-page links — because those live on the page itself. The outline tree does not live on the page, so nothing about a plain page-copy makes it come along. It has to be read from the old document, rebuilt entry by entry with corrected page references, and written into the new document's catalog — a distinct step that a page-copying merge doesn't perform.

Why this tool doesn't do it (yet)

ClientPDF's merge is built on pdf-lib, the same open-source library nearly all browser-based PDF tools use, because it's the one mature option that runs entirely client-side with no server round-trip. pdf-lib has no API for reading or writing a PDF's outline tree at all — not a partial implementation, a missing one. Any merge tool built on it, this one included, inherits that gap: it can copy pages faithfully, but it has no way to rebuild bookmarks even if it tried.

What still works after a merge

All pages and their visible content, unchanged.
Links that point from one part of a page to another part of the same source page.
Page order, exactly as you arranged the files.

What doesn't carry over: the bookmark/outline panel, and any named destinations the original PDF used for cross-document links.

What to do if you need the bookmarks

If the source files' bookmarks matter for navigation, merge with desktop software that explicitly lists bookmark or outline preservation as a feature — most full PDF editors do.
If you only need a handful of entries, merge here first, then manually add a short bookmark list in a tool that supports creating outline entries.
If the bookmarks are cosmetic and the page content is what matters, no action is needed — the merge itself is complete and correct, just without that navigation aid.

Try it yourself: merge a few PDFs and check — the pages and their content will be exactly right, and now you know why the bookmark panel might come up empty.