Normally, the digital certificates are issued by a Root CA (Certification Authority). If the Root CA that issued the signing certificate is not included in Adobe Trusted Identities, the digital signature is considered as not trusted when the document is opened in Adobe Reader.
However, this doesn't mean that the signature is not valid. This has nothing to do with the signing engine but with the Adobe certificate validation procedure.
The user can validate the signature if the Root CA is already installed on Microsoft Certificate Store. As an alternative, the recipient must manually add the Root Certificate of the signing certificate on Adobe Trusted Identities. Only a few Root CA's are considered trusted by default by the Adobe certificate validation engine. Recommend to refer a step by step process to validate signature.