It's easy to upload any existing PDF articles into your Papers library. You can click the "Add" then "Add Files" (top left corner next to the search bar), and select PDFs from your desktop.
Or, you can drag and drop single or multiple PDFs from your computer directly into your Papers library.
This video will walk you through the process.
You may also find these helpful:
Importing from other reference managers
Easy importing with our browser extensions
(updated April 26th, 2023)