To summarize if I want to use the PDF built-in signing I will need to convert my OpenPGP into a X.509 cert
otherwise I can simply use the OpenPGP file signing
OpenPGP is kind of like the opposite of that - it does a lot of things, and none of them particularly well. To quote:
PGP does a mediocre job of signing things, a relatively poor job of encrypting them with passwords, and a pretty bad job of encrypting them with public keys. PGP is not an especially good way to securely transfer a file. It’s a clunky way to sign packages. It’s not great at protecting backups. It’s a downright dangerous way to converse in secure messages.