If you're on a Mac, just open the PDF in Preview and then Save As to a different file name. This gives you a PDF visually identical to the original (with TOC, selectable text, etc) but with no protection.
Preview is the unsung hero of macOS. It does so much that Adobe Reader charges for: signatures, annotations, page rearrangement, plus it has the added bonus of being able to open obj and stl files.
It doesn’t do actual (cryptographic) signatures that Adobe charges for. It only does pretty pictures of handwritten signatures as an inserted object, a trivially falsifiable things.
