Based on community feedback Papers is excited to add the feature Read/Unread!

When an article is Unread in your library it will appear bold. Papers will automatically mark the paper as "Read" after it is opened in the web viewer.

Screenshot of ReadCube Papers library

To manually mark an article as Read or Unread head to the gear icon in the right-hand corner and select Read/Unread in the drop-down menu.

Screenshot showing how to mark an article as read in ReadCube Papers