If you don't have or need the full paper in your library, you can add any reference by clicking on the "Add" followed by "Add Manual Entry". In your library - these references will not have a paperclip beside the title meaning there is no PDF attached. 

Once you've filled in the metadata, click "save" and your reference will be added to your ReadCube Papers library.

At any time you can click on "Locate PDF" to download the PDF via your proxy or by purchasing the access through a publisher's website.