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

If it says "(SECURED)" in the window title when you open the file and it shows that there's a "Password Security" applied under File - Properties - Security, then the file is secured, whether or not the author is aware of it.

It could also be that the file is digitally signed, which will prevent it from being edited.