I think the subject speaks for itself, but I'll ask anyway. How do I implement PDF viewing on the clientside?  Thanks

That specific error message looks like the PDF decoder is not initialized when the asynchronous requests for the thumbnails are requested.  To fix this issue, you can add the PDF Decoder to the RegisteredDecoders on the Page Init event. I have a suggestion that may improve your performance for this situation (Once you get the PDF loading): ...