each year vs per annum

each year and per annum both are adverbs.

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

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

each year and per annum: 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 year.

each year (adverb)	per annum (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)