Signature Validation Status

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.