There are two ways to add documents to this panel:
When working with documents in other helper panels, right-click on any document and select Add to Favorites.
When working with a document in the Research tab, select Add to Favorites in the Next Steps pane.
You can add as many documents as you like to Favorites. As the list gets longer, a scrollbar is displayed that will allow you to traverse the list; the size of the panel remains the same.