There are 3 ways to open PDF file on Windows Phone:
Use "native" PDF viewer - Adobe Reader
To do that, you need a location URI of your PDF file, and then you can either use the HyperlinkButton
<HyperlinkButton Content="Click here to open PDF" NavigateUri="URI of your PDF" TargetName="_blank" Margin="10,60,0,0"/>
or you could use the WebBrowser task to browse to the PDF which will invoke the PDF viewer:
WebBrowserTask browser = new WebBrowserTask(); browser.URL = "URI of your PDF"; browser.Show();
Use ComponentOne control for viewing PDFs
http://www.componentone.com/SuperProducts/PdfViewerPhone/
This will cost you some money, but should work OK.
Write your own control for rendering PDFs :)))