The three icons on the Viewing Control Bar allow you to switch between Envelope View, Scan View, and Information View. The Download PDF link allows you to download or open the PDF in Adobe Reader if the item has been scanned. When in the scan view, the page you are currently viewing and the total number of pages will be shown in the middle of the bar. On the right of the bar is a slider that controls your zoom. You may zoom in and out on the image of the envelope or while viewing the scan.
To put the viewing pane into full screen mode, select the full screen button to the right of the zoom slider bar.
The control bar can be moved up and down on your screen to display more or less of your mail item list. Simply click and hold then drag and drop to where it is most useful for you.