Why are Firefox and Safari having problems with certain PDF files on the web?
Adobe's Safari plugin for their PDF Reader doesn't work in 64 bit mode (and they don't supply a Firefox/Mac plugin at all!).

Try this Adobe kb article to see if setting 32-bit mode for Safari will work for you.  I had similar isses a while back, and I had to reinstall Reader (after upgrading to Snow Leopard) after doing this tip.