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Why Adobe Acrobat says "signature validity is unknown"
Acrobat does not know how Sovrasign confirms signers, so it shows an unknown identity even when the document is unchanged. Check it with the Sovrasign verifier.
When you open a Sovrasign PDF in Adobe Acrobat or Reader, you may see a banner such as "At least one signature has problems" and, for each signature, "Signature validity is unknown". In the details, Acrobat also says the document has not been modified since it was signed.
This does not mean the document was changed or that the signatures are fake.
Why it happens
Acrobat trusts a signature's identity only when it comes with a certificate from a company on Adobe's own list. Sovrasign confirms signers differently: each signer confirms their email address with Google, Microsoft or Apple, and that confirmation is sealed into the PDF with a one-time key made for that signature. Acrobat does not know how to check this kind of evidence, so it marks the identity as unknown. It can still check that nothing changed.
What to do
Check the PDF with the Sovrasign verifier. It checks both that nothing changed and who signed. It is free, needs no account, and the file stays on your device.
Other PDF readers behave like Acrobat or show no signature status at all.