Why Does My PDF Signature Show as "Invalid"? (Causes and Fixes)

A red cross or a warning that the signature is invalid or that validity is unknown. These mean quite different things, and it is worth reading the message carefully: "unknown" is common and usually harmless, while "invalid" means something specific and more serious.

Common Causes

1. The signing certificate is not trusted by your reader — Much the most common cause. The signature is cryptographically fine, but the issuing authority is not in your reader's trust store, so it reports validity as unknown rather than invalid.
2. The document changed after signing — Any modification after the signature was applied breaks it. This is the signature doing its job.
3. The certificate has expired or been revoked — Certificates have validity periods. Without a trusted timestamp, a reader cannot tell whether the signature predated expiry.
4. There is no timestamp — A timestamp from a trusted authority proves when signing happened. Without it, validity becomes ambiguous once the certificate expires.
5. A self-signed certificate was used — Self-signed certificates are not backed by any authority, so no reader trusts them automatically.

Quick Checks to Try First

• Read the exact wording: "validity unknown" is very different from "invalid"
• Open the signature panel and look at who issued the certificate
• Check whether the document has been modified since signing
• Try the file in Adobe Acrobat, which has the widest trust store
• Ask the signer which certificate authority they used

Step-by-Step Solutions

Solution 1: Work out which message you are seeing

"Signature validity is UNKNOWN" means your reader does not recognise the issuer — the signature itself may be perfectly good. "Signature is INVALID" means the cryptographic check failed, which normally means the document changed after signing. These need completely different responses, so establish which one you have before doing anything else.

Solution 2: Add the issuer to your trusted identities

If the certificate came from a legitimate authority your reader does not know, you can add it to your trusted identities in Adobe Acrobat's signature panel. Only do this when you have independently confirmed the signer is who they claim to be — adding a certificate to your trust store is a security decision, not a formatting one.

Solution 3: Treat a genuinely invalid signature as a real finding

If the document was modified after signing, the signature has correctly detected it. Do not work around this. Go back to the signer, tell them what you are seeing, and ask for a freshly signed copy. For a contract or anything with legal weight, this is exactly the situation signatures exist to surface.

Solution 4: Use timestamped signatures when you sign

If you are the signer and recipients report problems, sign with a certificate from a recognised authority and include a trusted timestamp, which keeps the signature verifiable after the certificate expires. For everyday agreements where cryptographic identity is not the point, a straightforward signature on the page is often more appropriate — it is visibly a signature and carries none of this trust-chain complexity.

When None of the Above Works

If you have tried everything and still see the problem, the PDF may have been created with proprietary tools that produce unusual internal structures:

• Contact the document creator and ask for an unlocked or editable version
• Ask for the source file (Word, InDesign) if editing is legitimate
• For government or official forms, check if a fillable online version exists on the official website

How to Prevent This in Future

• Always work on a duplicate — keep the original locked version intact
• When creating PDFs yourself, only apply restrictions you actually need
• Test your PDFs in multiple viewers before distributing
• For editable documents, consider sending the source file (Word / Google Docs) rather than a PDF
• Related reading: How to Remove Digital Signatures from PDFs

Frequently Asked Questions

Is this a sign of a serious problem? It depends, and here the distinction matters. "Validity unknown" is usually a trust-store gap. "Invalid" means the document changed after signing and should be taken seriously.

Can I fix this on my phone? Yes. Open a mobile browser, go to an online PDF editor, upload the file, and try the operation. Many PDF problems that seem to require desktop software are actually just viewer limitations.

Will fixing this delete my document content? No. The steps above modify how the PDF is processed, not the underlying content. Always work on a copy to be safe.

How long does it take to fix? A trust issue is resolved in a few minutes. A genuinely broken signature requires a new signed copy from the signer.