The Viewer is the section of the application where files such as videos, JPGs or PDF documents can be opened and viewed. From here these items can be reviewed by users, given notes for active work, edited by the relevant workers and numerous other actions can be performed which will be broken down on this page.
In PIX you can either use the Viewer in it's full mode or you can make use of the Workspace Viewer panel in the Workspaces area of PIX depending on your preference and whether you'd like to view content whilst still being able to access the other panels on offer.