P4Web in Viewer mode is used strictly as a depot browser and document server.
You cannot sync or edit files using a P4Web in Viewer mode, nor can you edit client specs or user specs. In fact, you can't do anything that changes the state of files or metadata in the Perforce depot. Therefore, many of the shortcut buttons and menu choices you'd find in the control panel of P4Web in Standard mode won't be available when in Viewer mode.
For information on starting up P4Web in Viewer mode, see How to Start P4Web.