pen pal vs pen-friend

pen pal and pen-friend both are nouns.

Word	Noun	Adjective	Verb	Adverb
pen pal	Yes	No	No	No
pen-friend	Yes	No	No	No

As nouns, pen pal and pen-friend are synonyms defined as:

pen pal and pen-friend: a person you come to know by frequent friendly correspondence

pen pal (noun)	pen-friend (noun)
a person you come to know by frequent friendly correspondence	a person you come to know by frequent friendly correspondence