What does the message mean?
A PDF reader answers two separate questions about a signature. Does the certificate behind it lead back to one the reader trusts? And has the document changed since the signature was applied?
“Validity unknown” answers the first question. A PDF reader keeps a list of certificate authorities it trusts. A certificate issued by one of them leads back to that list. A self-signed certificate is its own issuer, so nothing on the list vouches for it, and the reader says it cannot tell.
| The question | What “validity unknown” tells you |
|---|---|
| Does the certificate lead back to one the reader trusts? | No. The reader cannot trace it to a certificate it has been told to trust. This is the only question the message answers. |
| Has the document changed since it was signed? | Nothing. The reader reports that separately, and for a KuduDoc-sealed PDF the public checker answers it. |
Will a KuduDoc-sealed PDF show it?
When the last signature lands, KuduDoc seals the file. It records a SHA-256 fingerprint of the exact bytes, adds a certification signature that allows no later changes, and records a timestamp anchor (what a seal records).
A PDF sealed by KuduDoc today shows “certified by KuduDoc” in Acrobat, because its certificate is issued by a certificate authority on Adobe’s Approved Trust List. Seals made before we moved to that certificate used KuduDoc’s own certificate, so Acrobat shows those as “validity unknown”; the public checker still compares them with the sealing record.
What doesn’t it mean?
- It does not mean the file was changed. That is a separate question, with its own answer.
- It does not mean the signature doesn’t count. It is a reader’s message, not a legal finding.
- It does not mean the file is unchanged or genuine either. To know whether a KuduDoc-sealed file is unchanged, check it.
How do you check a KuduDoc-sealed PDF?
- Open KuduDoc’s free checker. It is free and needs no account.
- Drop the PDF you received: the file itself, not a screenshot or a fresh export. Your device calculates its SHA-256 fingerprint and sends only that. The fingerprint, the time, your IP address and your browser are logged.
- Read the three lines: file comparison, record source and timestamp. There is no overall pass or fail.
Seal record 14 Mar 2026, 11:46
Your result Read each line
File comparisonUnchanged since sealing
Record sourceAccount seal
TimestampConfirmed
We confirm the date of sealing and that this file is unchanged since. We cannot confirm what the document says is true.
A sealed PDF carries its seal id and a QR code that opens the checker. Check the file, not a printed code. A seal id printed on a page can be copied onto any document. Only comparing the PDF itself shows whether it is unchanged.
Should I mark the certificate as trusted?
Acrobat lets you mark a certificate as trusted on your own computer. That changes what your copy of Acrobat displays for files signed with it. It does not tell you who sealed the file, whether it is unchanged since sealing, or whether what it says is true.
Anyone can make a self-signed certificate with any name on it, so a certificate taken from a file you received cannot show who made it.
What about a PDF that wasn’t sealed by KuduDoc?
The same message appears on many signed PDFs whose certificate your reader is not set up to trust. To see who a signed PDF’s certificate names, who issued it and whether the signed part has changed, check the signatures in any signed PDF.
For other ways to tell whether a PDF was changed, see how to check if a PDF has been edited.
Questions people ask
Does “validity unknown” mean the PDF was changed?
No. It means the reader could not trace the signing certificate to one it trusts. Whether the file changed is a separate question. For a KuduDoc-sealed PDF, the free checker compares the file with the sealing record.
Will a KuduDoc-sealed PDF show “validity unknown”?
Not a current one. Adobe Acrobat shows it as certified by KuduDoc, because the certificate is issued by a certificate authority on Adobe’s Approved Trust List. Seals made before that change used KuduDoc’s own certificate and do show “validity unknown”; check those on the public checker.
How do I make Adobe show the signature as valid?
For a KuduDoc-sealed PDF, you don’t need to. Marking a certificate as trusted changes what your copy of the reader shows; it tells you nothing new about the file. Check the PDF on KuduDoc’s free checker instead: it compares the file itself with the sealing record.
Does “validity unknown” mean the signature doesn’t count?
No. It is a reader’s message about a certificate, not a legal finding. KuduDoc signatures are ordinary electronic signatures. Whether a document needs something more depends on the kind of document.
What does “unchanged since sealing” mean?
That the file’s bytes match the sealing record exactly. A match means unchanged since sealing — a fact about the file's bytes, not about what the document says, who signed it or who owns anything.
What if the checker finds no matching record?
No match does not establish tampering. The checker cannot tell a never-sealed file from an altered one. Ask the sender for the sealed PDF itself — not a re-saved, re-exported or scanned copy.
Sources
- Validating digital signatures, Adobe Acrobat Helphelpx.adobe.comHow Acrobat reports certificate trust, and changes after signing, as separate results.
- FIPS 180-4, National Institute of Standards and Technologycsrc.nist.govSHA-256, the fingerprint a seal records.
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