I use my email certificate to sign my PDF documents.
Whilst the certificate is of course valid, it doesn’t appear automatically valid when opening up the signed PDF file because Comodo are not part of Adobe’s Approved Trust List.
There is a workaround, but that involves manually accepting the certificate which is laborious for users to do, and not something they might know how to, and, i my opinion, not something that should be required as Comodo is a well established CA.
CAs who are part of AATL are automatically verified online and rubber stamped good if all OK. An alternative is seeing the competition having a headstart
See the list
It would be great if Comodo-generated certificates could be part of this.
edited to add links