What the warning actually means
When Adobe Reader opens a signed PDF, it runs three validation checks: is the signing certificate still valid, has the document been changed since signing, and does the certificate chain back to a root authority Adobe trusts. The 'signature validity is unknown' message means the first two passed but the third failed. The signature exists, the file is unaltered, and the document is not legally invalid. Adobe simply cannot automatically verify the signer's identity using its default trust list.
Why it happens
Three causes account for almost every appearance of this warning. First, the signing certificate is self-signed, which is common when a developer signs PDFs locally with a one-off identity. Second, the certificate comes from a certificate authority that is not a member of Adobe's Approved Trust List, so Adobe has no reason to extend trust to it. Third, the signature has no embedded RFC 3161 timestamp, which can cause the signature to drop into 'unknown' the moment the certificate expires or revocation data goes stale.
How to fix it
If you control the signing pipeline, switch to a digital ID issued by an AATL member CA. Any signature backed by one of those roots is automatically trusted in Acrobat and Reader, and the warning bar turns to a green check without any per-recipient configuration.
For long-lived documents, also embed an RFC 3161 timestamp from a trusted Time Stamping Authority and produce a PAdES LTV (Long Term Validation) profile signature. The timestamp proves the signature existed while the certificate was still valid, and LTV bakes the revocation evidence into the PDF so verification keeps working years later.
If you cannot change the signing certificate (for example, the document is already signed and you are the recipient), open the signature panel in Acrobat or Reader, click the signing certificate, choose Trust, then Add to Trusted Certificates. This makes the signature validate on that machine only, so it is a workaround rather than a real fix.
Is the signature still legally binding?
Yes, in most cases. The Adobe warning is a UI trust display, not a legal compliance check. A signature can still be enforceable under ESIGN, UETA, and eIDAS when the warning shows, as long as the signing record captures intent, consent, attribution to the signer, integrity of the document, and retention of the audit trail. To remove the warning entirely, move to a signing pipeline that issues certificates chained to an AATL member.
Back to All Questions