When opening a signed PDF, Adobe Reader prompts 'At least one signature requires validating' or “at least one signature has problems'' in the signature panel and in the signature stamps (if any). This means that Adobe does not trust the Certificate Authority that issued the certificate, eg., FNMT Camerfirma, Firmaprofesional, AVANSI, etc) To solve this, configure Adobe Reader to trust the certificate issuer. Right-click on the signature which is causing problems and select 'Show signature properties'. A new window opens with the ''Signature properties''. Here are the details on the signature. Click on 'Show signer's certificate'. Next, the Certificate Viewer opens. We see the certificate issuer (in this case Avansi) and the certificate details. We open the 'Trust' tab, where we see that This certificate is not trusted . Click on 'Add to trusted certificates'. Adobe Acrobat may show a warning message after clicking the button above. Click 'OK'. The import contact settings window will be open. By default, the option 'Use this certificate as a trusted root' is selected in the Trust tab. Keep this option selected and click OK in the following windows. Go back to the Signature Properties window. Press 'Validate Signature'. Perfect! Adobe Reader already confirms that the signature is valid as it already recognizes all certificates from the trusted issuer. Please do this operation with all Certificate Authorities