on a lower floor vs downstairs

on a lower floor is not an adjective while downstairs is an adjective.

Word	Noun	Adjective	Verb	Adverb
on a lower floor	No	No	No	Yes
downstairs	No	Yes	No	Yes

As adverbs, on a lower floor and downstairs are synonyms defined as:

Other synonyms of on a lower floor include below, down the stairs.

on a lower floor (adjective)	downstairs (adjective)
	on or of lower floors of a building

on a lower floor (adverb)	downstairs (adverb)
on a floor below	on a floor below