memorandum vs souvenirs

memorandum

noun
  • A short note serving as a reminder. 

  • A brief diplomatic communication. 

  • A written business communication. 

  • A page in an annual publication honoring the memory of a person who died during the past year. 

souvenirs

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