Cannot add password to a signed PDF contract

[jnaps]

I downloaded a signed contract and need to password protect it in order for me to email it (as required by my HR team). However, I receive the error message 'Document cannot be saved. Bad parameter' when I try to add a password. It also states that 'You cannot change security on this document because the document is signed or certified.' Could someone please assist ASAP? This is for a new hire and I need a timely resolution.

[jnaps]

Here are my security settings on the document:

Attempt to change password protection via document properties:

Attempt to change password after clicking on the 'Protect' tool --> 'Protect Using Password' button --> Entering my password and pressing 'Apply'

[Bernd Alheit]

You can't add protection to a signed document. 

[Test Screen Name]

Adding a password would actually destroy the signature. Which would be a bad thing. 

[try67]

You need to do it the other way around. Add a digital signature field, then apply a security policy (making sure to allow signing!), then sign the file.

[MikelKlink]

As others have already mentioned, you cannot encrypt a non-encrypted, signed PDF, at least not without invalidating the signature.

The reason for this is that the signature does cryptographically sign the bytes of the PDF file (except the embedded signature of course) as is, it does not sign some normalized (e.g. decrypted if applicable) version thereof, see the sketch.

If you would encrypt the file contents, therefore, you'd change those signed bytes and so invalidate the signature.