I haven't used the viewer in production, i just found it today and made a form as an example to use a viewer in the app. So i have no idea how stable it is.
Charles, you are right that you can use System.Diagnostics.Process.start(pdfname) but it will show the file outside the application.
If you want it inside your application you will need a viewer. I attached a screenshot of the free PDF.NET viewer inside an SF app:
You can buy the ComponentOne PDF toolkit solo here:
http://www.componentsource.com/products/componentone-pdf-net/index.html
And here are more PDF tools:
http://www.componentsource.com/features/pdf/net-components/index.html
As a free alternative, maybe you can use the IText tools to fill your PDFs and then use a free viewer: http://www.codeproject.com/KB/applications/PDFViewerControl.aspx
I haven't used this, though, so I can't say how well they work.
/Teddy