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 ...

For example, you can generate public and private keys from Keybase, a PGP key database. On Keybase, you can also encrypt and decrypt messages and view the public keys of other PGP users.

PGP’s major attraction is that it uses public key cryptography, allowing you to exchange encrypted files without having to swap private, or secret key information with the other party.

Ars Staff PGP Keys Dan Goodin -----BEGIN PGP PUBLIC KEY BLOCK----- Comment: GPGTools - https://gpgtools.org ...

All you have to do is import your public PGP key into your Facebook settings and you're good to go.

Some Facebook users should soon be able to receive encrypted emails from the social networking site if they add PGP public keys to their profiles.