Hoople - 3:02 pm on Dec 9, 2011 (gmt 0)
I just checked the top document. The 'PDF version' is 1.5 (6.x). On my Windows7 PC I have Adobe Reader version 9. Over time the PDF version changed making older reader versions unable to read newer PDF version files. The PDF version doesn't track with the reader version further complicating the understanding of PDF usage.
It could be the users having difficulty have older PDF readers. They would get a message about the file being corrupt IIRC.
The easiest workaround would be to update your link to adobe to http://get.adobe.com/reader/otherversions/ [get.adobe.com] so they get just the reader and not the force-fed McAfee (where your link leads). It could be part of their frustration and stopping them from upgrading.
I would change your first two sentences to:
You will need a current version of the Adobe Reader to view these files. If you need to get it or upgrade your current version you can download it for free from Adobe Systems.
Not knowing your file uploading method leaves one guess --- FTP as 'text' may make them unreadable. FTP as 'binary' could fix it. Gather the file specifics (which link) and error message if this goes beyond the reader version update.
Oh, and welcome to WebmasterWorld AmandaGal!