PDF and XPS file formats are widely used across the world and you could need one day to display them inside your application. Today I would like to share with you a simple way to do so by using an open source solution: MuPDF (a multiplatform lightweight PDF and XPS viewer).
MuPDF is already working on a WinRT solution you can grab on this GIT repo: http://git.ghostscript.com/?p=user/mvrhel/mupdf.git;a=summary
I also used the work of Libreliodev and especially their advanced port of MuPDF for WinRT.
The result is a simple but really useful Windows 8 Modern UI app that is able to display a PDF/XPS file: http://www.catuhe.com/msdn/pdfreader.zip