Remove a PDF password

What removing the password changes in the file

An encrypted PDF cannot be parsed before it is decrypted: its strings and streams are ciphertext, including the compressed object streams that hold most of a modern file's structure. So the file is taken apart at byte level instead. Every object is found by its header, every string and stream inside it is decrypted with the file key, the security dictionary is dropped, and a fresh index is written around the result. What you get back is a rebuilt document, not your original with the lock taken off.

That rebuild has one effect worth knowing. A PDF that has been saved several times carries its own history: each save appends to the file and supersedes objects earlier in it. Rebuilding takes the current definition of every object and writes only that, so the superseded revisions — older versions of edited pages, previous values of form fields — go away with the password. For most documents that is a tidy-up. For a document someone will examine, it is a change to the file.

When you want something else

When this reports an encryption variant it cannot rebuild, qpdf --password=yourpassword --decrypt in.pdf out.pdf almost certainly can. qpdf implements every standard security handler, it is the implementation other tools are checked against, and it removes the encryption by rewriting only what encryption touched, leaving the rest of the document — its title, author and dates included — exactly where it was. It is also how you would do a folder of files rather than one.