The file may have been saved with a certificate.
The general rule is in order to clear a signature you need access to the private key that was used to create the signature. So, the first question is are you absolutely, positively sure that the person trying to clear the signature has the associated private key installed on the computer they are using to edit the file?
The other possibility is the form fields have been locked by a subsequent signer. Without seeing the file it's hard to know for sure.
Thanks, Steve, for pointing me in the right direction. The signer says that they used the same computer to sign the form, as they are now using to try to clear it. Is there a way that they can check or search for the private key that they used to sign the document (if they do not recall where they stored it)?
In general, what is the best practice for storing these types of files?
From what you explained, it sounds like the private key is no longer on the computer or perhaps expired (?) or clear signature would have been enabled, right?
Thanks for your reply. This is helpful.