Can someone describe how to encrypt a message using someone else's PGP public key on an iPad or iPhone? Can someone describe how to encrypt a message which isn't an email using someone else's pgp public key on iPad or iPhone?
iPGMail is not just for email and will do what you want. 

Its web page, https://ipgmail.com, includes this sentence:


The app supports key generation, public and private key import and
export, and both encryption and decryption of files or email messages.