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 your article including "files". Note if a supplement is already present, the number of additional files will appear in red.
Select the "files" icon to view the files associated with an article.
In order to add a new supplement to the article, while the "files" tab is selected drag and drop any additional PDFs directly into the blank white space available.
To delete a file from a reference, click the trash can icon on the right side of a file listing.