Perhaps nobody uses xps-files?
Well, I do. Not because they´re neat or nice or good or anything but because they´re there and Microsoft actually has support for them.
I use xps-s for previewing office documents since they can be opened in wpf and Office standard configuration allows for xps export.
And if trying to build an xps supporting application for the first time there could be a few humps in the road. For starters, it´s not really in the standard framework. Instead, you need to reference a dll named ReachFramework that has an XpsDocument class which can load your files.
The next steps, however, are quite simple, just uncompress the file using ReachFramework and assign the paginator source (actual document reader) to your DocumentViewer control:
using(XpsDocument document = new XpsDocument("filename", FileAccess.Read, CompressionOption.Fast)) control.Document = document.GetFixedDocumentSequence();