As suspected, the Cybook was not able to open the PDF downloaded from Google books. It even crashed and I needed to do a reset. But even if the PDF would be readable, or could be converted to make it readable, many books will probably suffer from the problem that the Cybook PDF reader cannot zoom.
Just found instructions for making ebooks from Google books under "similar threads" below (
Google Books -> eBook). But this seems to be a very tedious work.