I created a form with Acrobat XI Pro that Reader can fill-in and save. When I tried to encrypt the form with a password, I got the message ‘You cannot change security on this document because the document is signed or certified’. The filled-in form may contain sensitive data, though.
How do I encrypt a form with a password?
Did you Reader-enable the form before you attempted to apply security? If so, you have to reverse the order. First set up the security you want and then Reader-enable the document.