An invalid digital signature warning in a PDF can hold up a supplier payment, an onboarding file or a contract approval. Before requesting a new signature, open the signature details and read the exact message. Your PDF viewer may have found a change to the signed file, or it may lack the information it needs to verify the signer’s certificate.
Check what kind of signature the PDF contains
A visible signature can be a handwritten mark, an image or a typed name. Its appearance alone tells you little about whether the PDF contains a certificate-based digital signature. A digital signature uses a certificate to help a compatible PDF viewer check who signed and whether the signed content has changed.
Open the viewer’s Signatures panel to see whether it lists a certificate-based signature. Some signing workflows provide evidence separately, such as an audit record. If your process specifically requires a certificate-based signature and the panel shows none, ask the sender which signing method they used and request the original signed file.
Open the original PDF in a supported viewer
Download the file from the original signing request or your approved document repository. Keep that copy intact. Printing to PDF, merging pages or exporting a fresh copy can change the file or remove its signature data.
Open the original in an up-to-date desktop PDF viewer that supports digital signature validation. In Adobe Acrobat, open the Signatures panel and review the validation details. Use Signature Properties and Show Signer’s Certificate to inspect the certificate. If the PDF has several signatures, record which one shows the warning.
Find out what the warning means
| What the viewer reports | What to check |
|---|---|
| Changes after signing | Review the change details and, where available, compare the signed version with the current file. Some workflows permit later signatures or form entries. Check which changes the document allows and whether they match your process. |
| Untrusted signer certificate | The viewer cannot establish trust in the signer’s certificate or the certificates that support it. Ask IT to verify the issuer and certificate chain. |
| Revocation status unavailable | The viewer could not confirm whether the issuer withdrew the certificate. Ask IT to check whether the viewer can reach the issuer’s status service. Treat an explicit revocation result as a separate finding and escalate it. |
| Timestamp cannot be verified | Check whether the signature has a verifiable timestamp from a trusted service. IT can review the timestamp certificate and help interpret certificate validity dates in relation to the signing time. |
Suppose finance receives a supplier agreement with an unfamiliar certificate issuer. If the viewer reports no unexpected changes to the signed content, IT can focus on certificate trust. If the signed version shows a different payment amount from the current file, finance should hold the agreement and verify the approved version with the sender.
Record the details before you escalate
Capture the file name, agreement reference, source of the download and exact warning. Include the PDF viewer and version, the affected signature and, where visible, the certificate holder, issuer, serial number and timestamp status. Keep any screenshots and document copies in your approved support channel.
Contact IT for viewer settings, certificate trust, revocation checks or timestamp issues. Contact the sender if you received an exported copy, the signature is missing or the file differs from the agreement you expected. If the certificate itself needs investigation, ask IT to coordinate with the certificate issuer.
Keep the organisation’s verification settings in place while you investigate. Adding an unfamiliar certificate to the trust list can hide the issue you need to resolve.
Decide whether to accept, hold or escalate
- Accept when you have the expected agreement, the required signatures pass the checks in your policy and you can account for any permitted later changes.
- Hold when you lack the original file, the warning remains unresolved or you cannot confirm the signer’s certificate or timestamp where required.
- Escalate when you find an unexpected change, a revocation result or a mismatch between the signer and the expected approver. Route the issue to IT and the document owner before relying on the file.
A short review record gives the next person enough information to pick up where you left off. If your team is reviewing how it sends and tracks agreements, explore SignuluOne.