Do you have Acrobat Reader installed? I was going to say reinstall it, but there's been an update to 6.x so you might want to go ahead and get it.
Seriously, are you able to d/l PDF files from other sites without any problems? If you encounter the same problem, then I would say it's something messed up in either your file extensions or browser mime settings. Easiest thing would be (re)install Acrobat Reader.
Otherwise, email me and let me know which ones you want and I can email them to you. The files aren't that large.