How to check if a digitally signed PDF is valid, and what "validity unknown" means

To check a digitally signed PDF, open Verify PDF signature and press Open the verifier. The check opens on the myutils.me PDF signature verifier in a new tab, where you choose the signed PDF. It reports, for each signature, whether the document changed after signing and what the certificate chain says about the signer.

The PDF is checked in your browser there and is not uploaded.

Steps

1. Open Verify PDF signature and press Open the verifier. The myutils.me verifier opens in a new tab; this page stays where it is.
2. Choose the signed PDF on your phone or computer.
3. If the PDF has a password, enter it when asked so the signature can be read.
4. Read the result for each signature and the certificate details under it.

Why the check opens on another site

A complete signature check sometimes has to look things up: the issuer's public certificate when the PDF does not carry the whole chain, and whether a certificate was revoked before its expiry date. Those lookups need a small server, which this site does not run. The myutils.me verifier has one.

What leaves your browser for those lookups is limited: the address being fetched and, for a revocation check, the certificate's serial number and its issuer's identifiers. The PDF itself is not sent.

What a digital signature is

A digital signature in a PDF is a block of data inside the file. It records which bytes were signed, a fingerprint of those bytes, the signer's certificate, and a value that only the signer's private key could have produced. Checking it answers three questions, in this order:

1. Is the document unchanged since it was signed? The signed bytes are fingerprinted again and compared. One changed byte is enough to fail.
2. Does the signature verify? The signature value is tested against the public key in the signer's certificate.
3. Can the signer's certificate be traced to an authority that is trusted? Each certificate in the chain must have been issued by the one above it, up to a trusted root.

A picture of a handwritten signature, such as one added with the PDF editor, is not a digital signature. There is nothing in the file to check.

Reading the common results

Why your reader says "validity unknown"

Desktop and phone PDF readers show "Signature validity is unknown", or a summary line such as "At least one signature has problems", most often because of the third check. The file is intact and the signature is genuine, but the reader does not know the authority that issued the signer's certificate. That happens when:

• the issuing authority is not on the reader's built-in trust list,
• the trust list on the device is out of date, or
• a revocation check could not reach the network.

That is not the same as a forged signature. A changed or tampered document shows up as changed or invalid, which is the result to worry about. If you get it, do not rely on that copy; ask the sender for a fresh one.

What a check reports is what the certificate chain says. It does not decide whether a document is legally accepted, which depends on where you are and who you are dealing with.

Keep the signed original

A signature covers the exact bytes of the file. Any tool that saves a new copy, whether it compresses, merges, edits the metadata or adds a watermark, produces a file whose signature no longer verifies. Send the signed original when someone needs to check it. If you only need a picture of a signed page, save that page as a JPG.

Questions

Is my signed PDF uploaded?

No. The myutils.me verifier checks it in your browser. Only certificate addresses, serial numbers and issuer identifiers may be looked up, never the PDF.

Is the verifier free?

Yes, with no sign-up.

Is the verifier linked to a software maker or an authority?

No. It is an independent checker, not affiliated with any PDF software maker, certificate authority or government.

Can I check a PDF signed by several people?

Yes. The verifier shows each signature's result.