pen-friend vs pen pal

pen-friend and pen pal both are nouns.

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

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

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

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