IcePdf is the best that I've seen (that's free) for reading pdfs. JPedal is awesome, but not free.
If you're going to be generating images from pdfs that the general public can send you, I assure you (from experience) that you'll get pdfs that will crash the JVM. (ie: If they're many-layered pdfs with all vector graphics). This pdf is an example that will crash many libraries (but is a perfectly valid PDF without anything funny like Javascript, etc).
We've gone down the route of trying to use a multitude of libraries and eventually resorting to delegating the work of creating a thumbnail to ImageMagick, which is a highly optimized C program for image manipulation.
