When you select a reference in your library, the right side article details panel opens. At the top of this panel, you'll see other options for the article including the Files section.
Select the Files icon to view the files associated with an article and to attach new ones.
Clicking the thumbnail or title will open the file in the web viewer (in a new browser tab), if it is of a supported file type:
However, any file can be immediately downloaded by clicking the Download button next to the file listing.
To attach a file - whether a main article PDF or supplements - simply drag and drop them into the Files panel. A brief processing period, indicated by the "processing" status, is required before newly-imported files can be viewed in the ReadCube web viewer.
Please note, there is an overall file size limitation of 100MB.
To delete a file from a reference, click the trash can icon on the right side of a file listing.