each year vs per year

each year and per year both are adverbs.

Word	Noun	Adjective	Verb	Adverb
each year	No	No	No	Yes
per year	No	No	No	Yes

As adverbs, each year and per year are synonyms defined as:

each year and per year: by the year; every year (usually with reference to a sum of money paid or received)

Other synonyms of each year include annually, p.a., per annum.

each year (adverb)	per year (adverb)
without missing a year	by the year; every year (usually with reference to a sum of money paid or received)
by the year; every year (usually with reference to a sum of money paid or received)