Apps that allow External Display

Air Video, not sure about others as this is the one I use the most

 

There's an app called Projector that will display most document types, spreadsheets, powerpoints, etc. It doesn't allow editing or mark-up on the screen, but it does include a virtual laser pointer. By default, the presentation is necessarily in landscape mode. But it allows you to set the zoom-level should you prefer to display a full-page document at once, say, rather than breaking it up into two separate screens of information.