Guide · Updated · By Buğra Sözeri
How to Verify an Electronic Signature
How you verify a signature depends on what kind it is. Certificate-based digital signatures can be validated in a PDF reader. Simple e-signatures are verified through the audit trail and records around them.
Step by step
- Identify the signature type. Open the PDF in Adobe Acrobat Reader. If it shows a signature panel or a banner about signatures, it contains certificate-based signatures. If not, the signature is likely an image or annotation.
- Open the Signatures panel. For certificate-based signatures, open the signature panel in Acrobat Reader to see each signature and its validation status.
- Check the status. Look for whether the document has been modified since signing and whether the signer's certificate is trusted.
- Review the audit trail. For simple e-signatures, open the audit trail or certificate of completion from the e-signature service and check names, emails and timestamps.
- Confirm with the signer if in doubt. If anything looks off, contact the signer through a channel you already trust, not one given in the document.
Why image signatures are hard to verify
A drawn or pasted signature on a PDF is just a picture. Anyone could copy it, so it proves little on its own. The evidence comes from the process: who sent it, to which email, and when it was signed.
Keeping verifiable records
When you send documents with eSignSimple, an audit trail records the signing process so you can show who signed and when.
How to verify a document signed with eSignSimple
Open the verify page and upload the signed PDF, or enter its SHA-256 fingerprint. The check runs against the fingerprint we recorded when signing completed; if a single byte has changed, it will not match. Then read the certificate of completion at the end of the PDF: signer names, emails, timestamps and IP addresses. Our SHA-256 glossary entry explains why a fingerprint cannot be faked by editing the file.
Red flags that a signed document may have been altered
Fonts or spacing that differ on one page, a signature that looks pasted at a slightly different resolution, a certificate that lists a different page count, or a fingerprint that does not match. If you see any of these, request the original from the service that produced it. Background on what the trail records is in what is an audit trail, and are electronic signatures legally binding explains why this evidence matters in a dispute.
What if the document was signed in Word?
A signature inside an editable .docx cannot be verified in any meaningful way, because the file can change without trace. Ask for the signed PDF instead; how to electronically sign a Word document explains why PDF is the signing format.
Why does a signed PDF fail a fingerprint check?
A SHA-256 fingerprint changes if any byte of the file changes, so a mismatch means you are not holding the exact file that was completed. That is not always fraud. The most common innocent causes: someone opened the PDF in an editor or viewer and saved it again, which rewrites the file even if nothing visible changed; the PDF was printed to PDF or run through a compression tool to make it smaller; an email or document management system altered the attachment; or you are checking a different copy, such as a draft.
The fix is to go back to the original. Download the completed document again from the signing service, or ask the sender for the copy they received when signing completed, and check that one. If the original matches and the copy you were given does not, compare the two page by page to see what differs. If no copy matches, treat the document with caution and ask the other party to confirm the terms in writing.
How do you read Acrobat's signature results?
For certificate-based signatures, Acrobat Reader shows a banner at the top of the document and details in the signature panel. A green tick generally means the document has not changed since signing and the certificate is trusted. A yellow warning usually means the signature is intact but Acrobat cannot confirm the signer's identity, often because the certificate is not in its trust list. A red cross means the document was changed after signing in a way the signature does not allow, or the signature is broken.
Look at the details, not just the icon. A signed PDF can legitimately receive later changes, such as another person's signature or filled form fields, and Acrobat lists these revisions. You can usually open the version as it was at each signature to see exactly what that person signed. If the warning is only about trust, ask the signer which provider issued the certificate and check whether it is on a recognised list, such as the EU trusted lists for qualified certificates.
What if all you have is a scan or a photo?
A scanned page or a photo of a signed paper is the weakest form of signed document: there is no certificate, no fingerprint from a signing service and no trail. You can still look for obvious problems, such as mismatched page sizes, different paper backgrounds, or a signature that appears identical pixel for pixel to one on another document, which suggests it was pasted.
For anything that matters, the better step is to ask for evidence rather than to analyse pixels. Ask the signer to confirm by email from an address you already know, or ask them to sign again through a service that produces a certificate of completion. If you send documents yourself, sending them for signature in the first place means you never have to do this; see how to send a document for signature.
A checklist for teams receiving signed documents
For HR, finance or a legal inbox that receives signed documents from others, a short routine catches most problems. Is there a certificate of completion or a certificate-based signature? Do the signer names and emails match the people you expected? Is the signing date consistent with the deal? Does the fingerprint or signature validation pass on the copy you will store? Is anything missing, such as a countersignature or initials on a key page?
Store the copy you checked, not a re-saved or re-exported version, and note the date you checked it. If a document fails any step, ask for the original before acting on it, especially before paying an invoice or changing bank details because of a signed letter. The terms involved are explained in the glossary entries on document hash and tamper-evident seal.
Our take
Verification is not about the look of the signature; nobody can tell a real squiggle from a copied one. It is about two questions: is this the exact file that was signed, and who was the signing request sent to? A hash answers the first and the audit trail answers the second. If a signed document arrives with neither, treat it as a claim rather than proof, and ask for the certificate of completion.
Sources and further reading
Frequently asked questions
What does "signature validity unknown" mean in Acrobat?
Usually that Acrobat does not trust the certificate used to sign. It does not necessarily mean the document was altered.
Can I verify a signature added in Preview or Files?
Those add image-based signatures with no certificate, so there is nothing cryptographic to validate. Rely on how the document was exchanged.
How can I tell if a signed PDF was changed?
For certificate-based signatures, the PDF reader reports changes. For other signatures, compare against the copy and document hash recorded by the e-signature service.
Can I check a document signed with another service on eSignSimple's verify page?
No. The verify page compares against fingerprints recorded when documents were completed in eSignSimple. For documents from another service, use that service's own validation or its certificate.
Does re-saving a signed PDF break it?
It can change the file's fingerprint, so a hash check will fail even if nothing visible changed. Keep the original file as received and work on copies.
Why does Acrobat show a yellow warning on a valid signature?
Usually because it cannot confirm the signer's certificate is trusted. The document itself may be unchanged; check the signature details for the reason.
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