Open Adobe Reader program and it should display the version as it fires up. If not, on the HELP pull-down menu click "About Adobe Reader" and it'll tell you.
Version 6.0 or higher should read all pdf files just fine.
You can download the latest version here:
http://www.adobe.com/products/acrobat/readstep2.html Or maybe the file you're trying to open is knackered! Get someone else to try opening it to see if it's ok.