so far vs hitherto

so far and hitherto both are adverbs.

Word	Noun	Adjective	Verb	Adverb
so far	No	No	No	Yes
hitherto	No	No	No	Yes

As adverbs, so far and hitherto are synonyms defined as:

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

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

so far (adverb)	hitherto (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
used after a superlative
to the degree or extent that