We currently use a Flex-based preview component, therefore the PDFs have to be transformed to SWF before being displayed. The good news is that the final transform should be cached, so then it's just a download/bandwidth issue which can't really be avoided.
You could probably alter the web-preview Share components to use a different viewer for PDFs, but we haven't investigated doing that here. Please feedback if you do manage to get something going though!
Thanks,
Mike
