Possibly corrupted during transit across the internet, but if that exact same attachment (not a different copy of it) will open on another PC then you obviously can rule out corruption, so probably something wrong with the Reader that won't read it or some other error on that pc.
Bad RAM can often cause a file to be reported as corrupt when same file runs or reads ok on a different PC. (I know this from personal experience).
Only cure is to remove/replace the bad RAM module(s).
For confirmation, you can test the RAM by booting from a CD containing 'memtest86'
You can download the ISO image file to create a bootable CD from here: http://www.memtest86.com/download.html
Of course, if you did have bad ram in a pc it would usually cause errors with all kinds of files, not just pdf ones. But wouldn't do any harm to check it anyway.
Edited by pip22, 03 March 2008 - 01:54 PM.