downstairs vs on a lower floor

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

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

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

Other synonyms of downstairs include below, down the stairs.

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

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