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
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: