penknife vs pocket knife

penknife and pocket knife both are nouns.

Word	Noun	Adjective	Verb	Adverb
penknife	Yes	No	No	No
pocket knife	Yes	No	No	No

As nouns, pocket knife is a hypernym of penknife; that is, pocket knife is a word with a broader meaning than penknife:

penknife: a small pocketknife; originally used to cut quill pens
pocket knife: a knife with a blade that folds into the handle; suitable for carrying in the pocket

Other hypernyms of penknife include pocketknife.

penknife (noun)	pocket knife (noun)
a small pocketknife; originally used to cut quill pens	a knife with a blade that folds into the handle; suitable for carrying in the pocket