I've been looking around the internet for an example that shows how to encrypt a file with an existing PGP public key that was sent to me. The examples I find generate the key pair on the fly and then ...

So no one wants to touch this one, huh? Let me ask a simpler question then. A client wants to send me data, but insists that it must be encrypted. I'm like "cool". They say, "send me your public key".