PDF Digital Signature

There are two different things called "PDF signature" and they work very differently

One is a drawn or typed appearance — your name in a font, a scanned image of your handwriting, whatever looks like a signature on the page. That's what most people mean when they ask how to sign a PDF. The other is a cryptographic digital signature, which is a mathematical proof embedded in the file structure itself. You can verify a cryptographic signature; it proves the document hasn't been altered since signing and links the signature to a specific certificate. A drawn appearance does neither of those things on its own. This tool supports both: you can add a visual signature overlay to any page, and if you have a .p12 or .pfx certificate, you can embed a proper cryptographic signature that Adobe Reader or any conformant PDF viewer will validate with a green checkmark.

Self-signed certificates and CA-issued certificates produce different levels of trust

If you generate your own certificate to sign with, Adobe and other viewers will show the signature as valid but note that the certificate isn't trusted by a root CA. For internal documents shared within a team that's fine — everyone who needs to verify can import your certificate as trusted. For contracts with external parties, documents submitted to government portals, or anything where the other side won't manually import your certificate, you need a certificate issued by a recognized CA, which costs money and requires identity verification. The cryptographic strength is the same either way; the difference is whether verification is automatic for the recipient.