Document.Viewer

Document.Viewer is a cute application that will let you view any file formatted as FlowDocument, Xps, Html, rich text and traditional text.

It's as simple as useful. You only have to run the program and select the file you want to open. The only additional option is the one to copy content or to print it.

In short, if you don't want a complex application but a good way to view this kind of files, Document.Viewer can be really helpful.