notepad vs pad of paper

notepad and pad of paper both are nouns.

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

As nouns, pad of paper is a hypernym of notepad; that is, pad of paper is a word with a broader meaning than notepad:

Other hypernyms of notepad include pad, tablet.

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