PRC vs mainland China

PRC

noun
  • A person from the People's Republic of China, especially in contrast to native-born Singaporean Chinese. 

mainland China

name
  • Also used other than figuratively or idiomatically: see mainland, China. 

  • The People's Republic of China, viewed as the general geographical region administered by the PRC in the aftermath of the Chinese Civil War (as opposed to Hong Kong, Macau, and Taiwan which are coastal and insular areas) 

How often have the words PRC and mainland China occurred in a corpus of books? (source: Google Ngram Viewer )