Signatures marked as invalid in other PDF readers
PDF readers other than sign pro PDF, such as Adobe Acrobat or Nitro, may sometimes mark signatures entered via sign pro PDF as invalid.
This can happen if the signatures were created using a self-signed certificate.
The solution is to add the certificate to the Trusted Store for the programs which are validating the signatures.
To do this, right-click on the signature and select Properties, then click <Add to Trusted Contacts> as illustrated below.