You receive a notification when one or more files are shared with you. You can choose to deactivate these notifications (see the Manage my account and My settings sections).
If you missed a notification or turned off your notifications, you will find all the files shared with you since your last login in the Recent Files folder, located at the top of the Project’s Documents section.
You can also easily view these new documents in their respective folders thanks to a red dot that appears temporarily on the right corner of the document’s name.