we have tried all possible workarounds for this problem, does anyone know how to fix this issue?
What exactly is your problem with this?
The message points out two different facts:
- there have been subsequent changes to the document - if this message is a problem for you, retrieve and validate the originally signed revision of the PDF, not a later revision with changes applied after the signature.
- the signer's identity is unknown - if this message is a problem for you, either ask the signer to use a certificate issued by a certificate authority trusted by Adobe Acrobat by default (i.e. a CA from the AATL or EUTL) or - after confirming the certificate authenticity with the alleged signer - add the signer certificate to your trusted certificates in Adobe Acrobat.