Papers 3 introduces an easy way to import your existing Mendeley or Zotero library to Papers. Go to search mode and choose from Mendeley or Zotero import options. Papers asks you to log in to your Mendeley/Zotero account. Once you given permission to Papers to access your Mendeley/Zotero library Papers will import both the metadata and PDFs to your new Papers library. However, don't worry about loosing your Mendeley/Zotero library. They will still remain the same after the import has finished.
- You can also go to File > Import > Mendeley/Zotero library to import your library to Papers.