Search for "PDF import extension" (beta) for Open Office. It works ok although it messes up the layout sometimes. Install the extension, then open the document in Draw. From there you can add links (Insert ¦ Hyperlink), then export as PDF to save the new version.
If you have access to the source code of the PDF files, then simply edit the source. If you don't have access to the source code, making changes may violate copyright. If the source code has been lost, if possible and practical, it should be recreated.