Guide
How to verify a signed PDF
Short answer
Open the PDF in Adobe Acrobat Reader and check the Signature panel: a digitally sealed file shows who sealed it and whether the document has been modified since. For a document signed through Sign PDF, drop the file into signpdf.ai/verify; it is fingerprinted in your browser and compared with the sealed original, and the file never leaves your computer. A PDF with only a pasted signature image has no seal, so it cannot be verified this way.
Updated 25 October 2026
Step by step
- 1
Open the PDF in Acrobat Reader
Use the free Adobe Acrobat Reader rather than a browser or Preview, which do not check signatures.
- 2
Open the Signature panel
Click the signature bar at the top or the Signature Panel button.
- 3
Check for changes
Confirm the panel says the document has not been modified since it was signed.
- 4
For Sign PDF documents, go to signpdf.ai/verify
Scan the QR code on the certificate or visit the page directly.
- 5
Drop in the file
Your browser fingerprints it locally. The file is not uploaded.
- 6
Read the result
A match shows the document, sender, signers and completion time. No match means the file differs from the sealed copy.
Why a signed PDF can be altered
A PDF is an editable file. If someone signs a contract by stamping their signature image on it, anyone with a PDF editor can later change the price on page two and nothing about the file shows it. Two copies of 'the signed contract' can differ, and neither proves which is right.
A digital seal fixes that. The signing service calculates a cryptographic fingerprint of the finished file and signs it with its certificate. Change one character and the fingerprint no longer matches.
Checking the seal in Adobe Acrobat Reader
Open the file in the free Acrobat Reader. A bar at the top shows the signature status; click Signature Panel to see the details. Look for two things: who applied the signature and when, and whether the document has been modified since it was signed.
Reader may say the signature's validity is unknown if the certificate that sealed it is not on Adobe's own trust list. That is about trust in the certificate, not tampering. The key line is whether the document has been altered since signing. If it has, Reader says so plainly.
Preview on a Mac and most browsers do not check PDF signatures, so use Reader for this.
Verifying a Sign PDF document
Every document completed through Sign PDF has a QR code on its certificate of completion that leads to signpdf.ai/verify. Anyone can use it, without an account. Drop the PDF in; your browser calculates its SHA-256 fingerprint locally and checks it against the fingerprints we recorded.
If it matches the sealed copy, the page says so and shows the document title, envelope reference, sender, completion time and signers' names. It also recognises the original unsigned upload. If it says 'No match', the file has been edited, printed and rescanned, or was signed elsewhere. Ask the sender for the copy we emailed them.
The certificate itself is a second check. It lists the SHA-256 fingerprint of the original document and an audit trail where each entry is chained by hash to the one before, so entries cannot be removed or reordered quietly.
What verification cannot tell you
A matching file proves it is the one that was sealed. It does not prove who was behind each email address. For that, read the certificate: verified email, IP address, approximate location, device and times for each signer. If you have doubts about identity, contact the signer by a known channel. We do not check government ID.
For PDFs signed with other services, use their own verification tools or Acrobat Reader's Signature panel.
Questions people ask
How can I tell if a signed PDF has been edited?
If it was digitally sealed, Acrobat Reader's Signature panel tells you whether it has been modified since signing. If it only has a signature image, there is no reliable way to tell from the file.
Is my file uploaded when I use signpdf.ai/verify?
No. The fingerprint is calculated in your browser and only that fingerprint is compared. The PDF stays on your computer.
Why does Acrobat say 'validity unknown'?
Usually because the certificate that sealed the file is not on Adobe's trust list on your computer. Check the separate line on whether the document has been modified.
Can I verify a printed copy?
No. Printing and rescanning creates a new file with a different fingerprint. Verify the original PDF that was emailed to you.
Can I verify a DocuSign or Adobe-signed PDF here?
No, signpdf.ai/verify only recognises documents sealed by Sign PDF. Use Acrobat Reader's Signature panel or the other service's own tool.
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