You have three routes:
- Launch the pdf, which would cause it to be opened in another application
- Pay for an already implemented control, such as this one. Note how they bundle a bunch of stuff together, so you won't just get a pdf viewer
- Make your own pdf viewer, which is definitely the hardest way.
Edit: How to launch an app associated with a file:
StorageFolder folder = ApplicationData.Current.LocalFolder;
StorageFile file = await folder.GetFileAsync(<filename>);
Launcher.LauncheFileAsync(file);
You can call this from wherever you need it.