Check the digital signatures on a signed PDF. The tool reports who signed the document, when, which certificate was used, and - most importantly - whether the file has been modified since the signature was applied. Use it when you receive a signed document and need to confirm it is genuine and intact before acting on it.
The signature is cryptographically sound, but the certificate does not chain up to an authority in the trust store. That is the normal result for a self-signed certificate and does not by itself mean anything is wrong.
Content changed after signing. Sometimes that is innocent - a later signer adding their own signature - but if a single signature covers the file and the document reports as altered, do not treat it as authentic.
A signature added as a picture with the Sign PDF tool is not a digital signature and there is nothing here to verify. Only certificate-based signatures can be validated.
Files are uploaded over an encrypted connection and processed on CoonTool's own server, not in your browser. The working copy is deleted automatically once the job is done, and in any case within 24 hours. Nothing is stored permanently and no account is required.