How does Qiqqa compare to Zotero? According to an old (2010) post by the author of Qiqqa (https://forums.zotero.org/discussion/13326/qiqqa-and-pdfs/):

  It's Jimme here. I am the guy building Qiqqa.

  Qiqqa is not trying to be a reference manager like Zotero or Mendeley. 
  As I do my PhD I am building something that is excellent at helping me (any anyone else who wants to use it)
  READ, UNDERSTAND and DIGEST the papers in my library using Computational 
  Linguistics techniques, and not LOCATE/DOWNLOAD/COLLECT the papers.

  Zotero's incredible strength lies with its ability to download reference matter from hundreds of sources - and that is something Qiqqa would never need to compete against. 
  It is great to see a community at work writing the hundreds of parsers and scrapers of the various online libraries.