If you need to display a PDF document without the use of any plug-ins, try rendering each page to an image format like JPEG or PNG.
Converting PDF documents to Flash SWF is another solution that seems to work well for the web. Scribd provide a document sharing service that does this, and there are others, so it's worth looking around.
If you want to cut your own in VB.NET, the documentation for ABCpdf .NET includes a freely adaptable project which you could use as a starting point. See example projects > swfexport.