Signing PDFs digitally is kind of a pain, though. There are several different standards, compatible with different readers, and you need to pay up to get a trusted certificate or Adobe will stick a huge "THIS DOCUMENT WAS SIGNED BY AN UNKNOWN PARTY" banner across the screen when you open it. And then you need to opt into the services of a timestamp server to validate your signature or your document might be marked untrusted when your certificate expires.

It's a better technical solution but unfortunately it's not as simple as you'd hope it to be.