Digital Signatures - Automatically Trusting Signers Certificate

Hello,

I was wondering if there was a way to have certificates be automatically trusted after they're signed with a digital signature using a self-signed certificate. Currently all of our signed PDFs have a message at the top of them saying "At least one signature has problems.". I understand how to trust each signer and "verify" the signature, but it there a way to prevent this warning entirely ?  Does purchasing a certificate make any difference ?

Thanks in advance!