down the stairs vs downstairs

down the stairs is not an adjective while downstairs is an adjective.

Word	Noun	Adjective	Verb	Adverb
down the stairs	No	No	No	Yes
downstairs	No	Yes	No	Yes

As adverbs, down the stairs and downstairs are synonyms defined as:

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

down the stairs (adjective)	downstairs (adjective)
	on or of lower floors of a building

down the stairs (adverb)	downstairs (adverb)
on a floor below	on a floor below