One way to get Revit into Viewer mode is simply to allow the 30 day trial to expire, however Revit does have a viewer (that does not of course use a license) and can be accessed from the Windows Start Menu:
The Viewer mode interface looks no different than the actual Revit interface as it is simply an unlicensed Revit that will allow a user to navigate the model and sheets as they like.
An important distinction about Viewer Mode is that it cannot be used to save (or save as) or to export in any way, it can however be used to print so long as nothing has been modified in any way. This ability to print can be helpful if the scenario arises that the Revit user is unavailable and Engineering needs to run the prints in their absence. (Again this is fine so long as Engineering does not make any changes, as soon as a change is made the print functionality is lost. In such an instance simply close the file – remember you can’t save – and re-open the same file and send it to print.)