pad of paper vs notepad

pad of paper and notepad both are nouns.

Word	Noun	Adjective	Verb	Adverb
pad of paper	Yes	No	No	No
notepad	Yes	No	No	No

As nouns, notepad is a hyponym of pad of paper; that is, notepad is a word with a more specific, narrower meaning than pad of paper:

Other hyponyms of pad of paper include message pad, writing pad.

pad of paper (noun)	notepad (noun)
a number of sheets of paper fastened together along one edge	a pad of paper for keeping notes