It's easy to upload any existing PDF articles into ReadCube Papers library. You can simply drag and drop PDF files directly into your ReadCube Papers screen. You can also click the "Add" then "Add Files" (top left corner next to the search bar) and select PDFs from your desktop.
You can also drag and drop single or multiple PDFs from your computer directly into your ReadCube Papers library.
This video will also walk you through the process.
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