If you're looking to move your library (PDFs only or PDFs with metadata) our of ReadCube Papers, we offer several different options.
To export only the metadata of your library, head to the desktop application and navigate to the Options of the library you're looking to export:
There, you'll see the option to export your contents of your library as a .bib, .ris, or .csv file.
To export your PDFs out of ReadCube Papers, head once again to the desktop application, into the Options of the library in question, to PDF Downloading and mark your PDFs as "available offline" (this may have already been selected - no worries! Read on):
Once you've marked the PDFs in your library as "available offline", the desktop application will begin to download a local copy of all of your files. You will know which file is locally available by the presence of a paperclip in the library view:
In the above example, the top reference has a local file attached to it while the second does not.
Once the download of all of your files has been completed (this can take some time, especially for larger libraries) all of your PDFs will be available within your Papers Library folder. By default, this folder is located within Documents on your computer (Mac and PC) but if you're unsure of where your library is located, launch the desktop app and head to Papers Settings - Library, and you'll see the location of those PDFs:
With those PDFs downloaded to a single location, you can now easily move them to the service of your choice.