How do I remove security from "encrypt". I don't have a password. I can open the document but I can't put stamp on it in "annotations". The sender is not aware of any security placed on the document

How do I "remove security" from "encrypt".  I don't have a password.  I can open the document but I can't put stamp on it in "annotations".  The sender is not aware of any security placed on the document.

Thank you

Dave