up to now vs so far

up to now and so far both are adverbs.

Word	Noun	Adjective	Verb	Adverb
up to now	No	No	No	Yes
so far	No	No	No	Yes

As adverbs, up to now and so far are synonyms defined as:

up to now and so far: used in negative statement to describe a situation that has existed up to this point or up to the present time

Other synonyms of up to now include as yet, heretofore, hitherto, thus far, til now, until now, yet.

up to now (adverb)	so far (adverb)
used in negative statement to describe a situation that has existed up to this point or up to the present time	used in negative statement to describe a situation that has existed up to this point or up to the present time
prior to the present time	used after a superlative
	to the degree or extent that