Merge PDF

What survives a merge is the part most tools decline to state. The pages themselves come through untouched: text stays selectable and searchable, images keep their resolution, and nothing is re-rendered or re-compressed. What does not come through is everything that belongs to the document rather than to a page.

That distinction is not a policy, it is the shape of the file format. A PDF keeps its outline, its form definition, its accessibility structure tree, its embedded attachments, its page labels and its metadata in one catalogue at the top of the document, and merging builds a fresh catalogue containing only pages. So bookmarks disappear, form fields stop being fields even where the boxes still draw, a tagged and accessible document comes out untagged, and the title and author fields are replaced with the merging library’s own name and today’s date. Internal links are the case that surprises people, because they look as though they should survive: the link annotations do get copied, but each one carries a reference to the page it pointed at, and that reference is copied alongside rather than redirected, so the link ends up aimed at an orphaned copy of the old page and goes nowhere. Reinstate anything you need in the finished file.

A digital signature is a different kind of loss and is not recoverable at all. A signature covers a byte range of one specific file, so the moment a new file exists there is nothing for it to certify. Sign the merged document rather than merging signed ones.

Encryption is the case to know about before you try it, because it fails without saying so. The parser these pages use contains no decryption at all. It is asked to ignore encryption, and that suppresses the error rather than unlocking anything: an encrypted file loads, the page count comes out right because the document structure is readable, and every page’s content stream stays encrypted. The merged result is then a fraction of the size it should be, with pages that render blank or broken in every reader, and the panel reports it as a success. That is equally true of a file which only forbids printing or editing and of one which needs a password to open. Take the protection off first, in whatever produced the file, and merge the unrestricted copy.