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):
If you're loading a stream or byte array into the WebThumbnailViewer and/or the WebImageViewer, these controls will automatically cache it to disk in the format that you gave it to them, so it can bring back the pieces while it's asynchronously loading tiles or thumbnails. (If you load the same stream in both controls, you will be caching two separate copies of the same image)
You can improve performance a great deal by opening that stream or byte array into AtalaImages, one page at a time using the TiffFile class to encode it as a Tiff, and saving that image to the ImageCache. Now, since you have a local copy, you can open the same image from disk with both controls. This will also speed up the tile loading, as our Tiff decoder is optimized for regional reading, and our PDF decoder is not.