Click on Upload Files and make sure to select valid PDF files under 200MB from your device. Make sure your PDF is not password-protected. You can upload one or many references at once.
Next you will see information about the file you are trying to upload, if the file format is valid, you should see a new button: Add Reference. Click on it to confirm the upload.
You should now see your past reference in your library. Please wait for the document to upload and parse. (You can see the status on the document on the bottom left of its preview image)