mainland China vs People's Republic of China

mainland China and People's Republic of China both are nouns.

Word	Noun	Adjective	Verb	Adverb
mainland China	Yes	No	No	No
People's Republic of China	Yes	No	No	No

As nouns, mainland China and People's Republic of China are synonyms defined as:

mainland China and People's Republic of China: a communist nation that covers a vast territory in eastern Asia; the most populous country in the world

Other synonyms of mainland China include Cathay, China, Communist China, PRC, Red China.

mainland China (noun)	People's Republic of China (noun)
a communist nation that covers a vast territory in eastern Asia; the most populous country in the world	a communist nation that covers a vast territory in eastern Asia; the most populous country in the world