Adobe Acrobat "The Signer's Identity Is Unknown" — Why It Happens and How to Fix It

Symptom fingerprint

The exact strings, error codes, and UI surfaces that map to this issue:

What this message actually means

Adobe Acrobat checks two separate things when it opens a signed PDF: whether the signed bytes were altered (integrity) and whether the signing certificate chains to a root Acrobat trusts (trust). "The signer's identity is unknown" is purely the second check — trust — reporting that Acrobat reached the certificate but found no trusted anchor above it. It is not saying the document was tampered with.

That's the key thing to understand, because it means the fix is almost never re-signing the document. You either tell Acrobat to trust the certificate (or its issuer), or you refresh the Adobe Approved Trust List (AATL) so the issuer is recognized automatically. I see people panic at the yellow banner and ask the signer to redo everything — nine times out of ten that's wasted effort.

Why this happens

Three situations produce this exact banner:

• The signer used an internal, self-issued, or organizational certificate that was never meant to appear on a public trust list.
• The signer used a commercial certificate (Entrust, GlobalSign, DigiCert, IdenTrust) but your AATL is stale, so Acrobat doesn't recognize the issuer yet.
• A US Federal PIV/CAC or other government certificate that chains to the Federal PKI rather than the commercial AATL — Acrobat won't trust it until you add the Federal root.

Fix 1 — Trust the certificate directly

Use this when the signer used an internal, self-issued, or organizational certificate that will never be on the AATL.

[IMAGE: Signature Properties → Show Signer's Certificate → Trust tab with "Add to Trusted Certificates" highlighted — alt text: "Adobe Acrobat certificate viewer Trust tab with Add to Trusted Certificates button"]

• Open the signed PDF and click the signature, or open the Signatures panel on the left.
• Right-click the signature → Show Signature Properties → Show Signer's Certificate.
• Go to the Trust tab → Add to Trusted Certificates.
• Tick "Use this certificate as a trusted root" and, if appropriate, "Certified documents", then click OK.
• Right-click the signature again → Validate Signature. The banner should turn green.

Fix 2 — Update the AATL / EUTL trust list

Use this when the signer used a commercial certificate that should already be trusted — Acrobat's trust list is just stale.

[IMAGE: Preferences → Trust Manager with the "Update Now" button under Automatic Adobe Approved Trust List updates — alt text: "Adobe Acrobat Trust Manager preferences showing the AATL Update Now button"]

• Edit → Preferences → Trust Manager (Windows) or Acrobat → Preferences → Trust Manager (macOS).
• Under "Automatic Adobe Approved Trust List (AATL) updates", click Update Now.
• Confirm "Load trusted certificates from an Adobe AATL server" is enabled.
• Close and reopen the PDF, then re-validate the signature.

Fix 3 — Let Acrobat use the OS trust store (Windows)

If your organization distributes its root CA through Windows, tell Acrobat to honor it.

• Preferences → Signatures → Verification → More.
• Enable "Validating Signatures" and "Validating Certified Documents" against the Windows Certificate Store.
• This doesn't apply on macOS — Acrobat there doesn't read the login Keychain by default, so use Fix 1 or Fix 2 instead.

Still not working?

If you've trusted the certificate (or refreshed the AATL) and the banner still says the identity is unknown, the usual culprits are a broken or incomplete certificate chain — an intermediate CA is missing — or a revocation/timestamp check that can't reach the CA's servers. A government or bridge-CA certificate can also need the Federal root added by hand before anything else takes.

[IMAGE: the yellow "at least one signature has problems" validation banner above a signed PDF — alt text: "Adobe Acrobat yellow signature validation banner reading signer's identity is unknown"]

This is a common one to hand off — on a screen-shared session I can inspect the exact chain, import the missing intermediate or Federal root, and confirm the green tick sticks across your machines rather than just on this one.

You might also need

More fix guides in Adobe Sign and related areas.