Signatures
How to check if a PDF is digitally signed, and if the signature is valid
A name in handwriting at the bottom of a page proves very little. A digital signature can be checked by anyone, for free, in a couple of minutes. Here is how, on a computer, online and on an iPhone.
Open the PDF in a program that validates signatures, such as the free Adobe Acrobat Reader, and look at its signatures panel. If the panel lists nothing, the PDF has no digital signature, whatever the page shows. If it lists a signature, check three things: that it’s valid, who the certificate says signed, and whether anything changed after signing. Online validators do the same check, but you have to upload the file.
Signed, or only signed-looking?
There are two very different things people call a signature in a PDF.
- A drawn or typed signature is a picture or an annotation on the page. Anyone can copy it to another document. It can still be a valid electronic signature in law, depending on the country and the agreement, but the file itself proves nothing about who placed it.
- A digital signature is data inside the file: a fingerprint of the signed bytes, sealed with the signer’s certificate. A validator can recalculate it and tell you if the document changed and whose certificate was used.
The two can look the same on screen, because a digital signature usually has a visible stamp too. The only reliable test is a validator: a digital signature appears in its signatures list; a picture doesn’t.
Verify it in Acrobat Reader (Mac or Windows)
Acrobat Reader is free and is what many recipients use, so it’s worth knowing what it shows. Adobe’s own steps:
- Open the PDF and choose Signatures in the right-hand panel.
- In the Signatures panel, choose Options, then Validate Signatures.
- Read the Signature Validation Status box.
- For the signer’s details, choose Signature Properties, then Show Signer’s Certificate.
With several signatures, All tools › Use a certificate › Validate all signatures checks them in one go. If the result says “unknown” or a blue bar says “At least one signature has problems”, the file isn’t necessarily forged: our guide to why a PDF signature shows as invalid or unknown goes through each cause.
Verify it online
Two public validators are run by governments rather than by software sellers:
- DSS demonstration, from the European Commission (ec.europa.eu/digital-building-blocks/DSS/webapp-demo/validation). It is the reference tool for European PAdES signatures and gives a detailed report. Its page says uploaded files are not retained, and that it is a demonstration meant for testing.
- VALIDe, from the Spanish government (valide.redsara.es). It checks signatures made with the certificates accepted by Spain’s @firma platform, such as FNMT and DNIe.
Both mean sending the document to a server. That’s fine for a public contract template, less so for a medical report or a bank statement. For those, verify on your own device.
What to check besides “valid”
A green tick answers one question: that the signed part hasn’t changed and the certificate checks out. Four more are worth a look.
| Check | Where it shows | Why it matters |
|---|---|---|
| Who signed | The certificate’s name and issuer | “Valid” means a certificate signed it, not that it’s the person you expected. A self-signed certificate can carry any name. |
| When | Signing time, and whether it’s timestamped | Without a timestamp, the time comes from the signer’s own computer clock, as Adobe notes. A timestamp from a trusted server proves it. |
| Changes after signing | The list of later revisions or changes | Filled fields and extra signatures are normal. A changed page is not. How to tell if a PDF was edited. |
| Certified or approval | The signature type | A certifying signature sets what later changes are allowed; an approval signature only signs. |
On an iPhone or iPad
Seeing a signature stamp on a phone screen tells you nothing about its validity. On a phone the choice is the same as above: upload the file to an online validator, or use an app that verifies on the device.
Keepsign verifies signatures on iPhone, iPad and Mac, on the device and free for one file at a time. Each signature gets one of four plain statuses: valid, valid with permitted later changes, not valid, or couldn’t be verified, with the reason in words. It shows who signed, when (and whether that time is proven by a timestamp), the PAdES level, the certificate chain and every change made after signing, page by page. Nothing goes online unless you tap Check online, and it tells you first what it will ask and to which server. We checked it against public test files from the European Commission’s DSS test bank: on the 20 with a documented expected result, it agrees with DSS on 19, and on the other, a known spoofing attack, it is stricter. See what changed after signing.
Sources
- Adobe, “Validate digital signatures”, helpx.adobe.com/acrobat/desktop/e-sign-documents/manage-digital-signatures/validate-digital-sign.html, updated September 23, 2025, read November 5, 2026.
- Adobe Community, “At least one signature has problems”, community.adobe.com/t5/acrobat-reader-discussions/at-least-one-signature-has-problems/td-p/12661549.
- European Commission, DSS Demonstration WebApp, ec.europa.eu/digital-building-blocks/DSS/webapp-demo/validation, read November 5, 2026.
- Gobierno de España, VALIDe, valide.redsara.es, read November 5, 2026.
- ISO 32000-1:2008, section 12.8, Digital signatures (public copy at opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/PDF32000_2008.pdf).
- Keepsign verifier tests, November 2026: 53 public signed PDFs (50 from the DSS test bank, 2 from Spain’s BOE, 1 from the U.S. GPO).