title_body: PDF Compressor online / software
upvoted_answer: I'm not sure what you mean by best. Best is subjective and dependent on various factors such as type of PDF document and acceptability of quality loss.
  
  However, Adobe Acrobat Pro DC offers an option to Reduce File Size.
  
  
  
  Similar software also offer such an option. Example: ABBYY FineReader (p. 3 of Source).