PDF files are Adobe Acrobat files - you need the Adobe Reader program to be able to display them. If you opened them in Word, Notepad, etc, then yeah, they'll display as a bunch of gibberish.
I assume you're using Windows, so go to
http://get.adobe.com/reader/ to download and install Reader to be able to view PDF files.
If you already downloaded it, if you double-click on a PDF file does it open up the Reader application to display it? If not, maybe you got the association of the .pdf extension messed up so it is trying to open .pdf files with Word instead of Reader. If you correct that, it should display the files correctly.