When I open a signed PDF, Adobe Acrobat Reader shows an error message “At least one signature requires validating” or “At least one signature has problem”, visible in the signature panel and signature stamps (if any). It means that the Certification Authority that issued the certificate (eg FNMT Camerfirma, Firmaprofesional, AVANSI …) is not a trusted issuer for Adobe Reader
To fix it, configure Adobe Reader to trust the certificate issuer. Press the right button on the signature with problems, and select “Show Signature Properties.“
A window with the PDF electronic “Signature Properties”
is opened. We can see the detailed information of the signature. Click on the button “Show Signer´s Certificate”.
Then, the Certificate Viewer is opened . On the left you can see the certificate issuer (in this case AVANSI) and details of the certificate.
Then, open the tab “Trust”, where you will see that “This certificate is not trusted” (this is the origin of the problem). Click on “Add trusted Certificates”
Adobe Acrobat may show a warning message after clicking the button above. We proceed to click “OK”.
The import contact settings window will be open. By default, on “Trust” , the option “Use this certificate as a trusted root” is selected, (in other versions, the message may be different). Keep this option selected (or select it if not) and click OK on the window that come out.
Go back to the Signature Properties window. Below there is a button that says “Validate Signature”. Press it.
Adobe Reader says signature is valid, all trusted issuer certificates are now recognized. This operation should be done with all Certification Authorities certificates you are going to use.