You have to use the LaunchFileAsync
method of Launcher
class. Example:
// Access the file.
StorageFile pdfFile = await Windows.ApplicationModel.Package.Current.InstalledLocation.GetFileAsync("file.pdf");
// Launch the pdf file.
Windows.System.Launcher.LaunchFileAsync(pdfFile);
You will find more info here:
Auto-launching apps using file and URI associations for Windows Phone 8