PDF-Recover can be used to decrypt protected PDF files, which have "owner" password set. If you can't remember your password, PDF-Recover will allow enable you to open decrypted files in any web browser with Acrobat Reader - without any restrictions. PDF-Password creates a 1:1 copy of your PDF-file without any password limitations, that means you can set edit, copy, print and so on.
Also a 128-bit encryption is supported by PDF Password.
OK, so you want to stamp your document. Maybe you need to give reviewers some advice about the document's status or sensitivity. This tip from author Ted Padova demonstrates how to add stamps with the Stamp Tool along with related comments.