penders - 4:03 pm on Dec 9, 2011 (gmt 0)
...so they get just the reader and not the force-fed McAfee (where your link leads)
It seems that this link also prompts with the additional McAfee checkbox, once you've actually picked an Adobe Reader version to download.
Are the users actually downloading the file, but failing to open it? Or failing to even download it? I was assuming the later?
Another thought... if their browser is configured to download automatically to a certain folder and not prompt (as I believe Chrome and Safari are set up by default?), then they may not realise they have downloaded the file, or where it might be located?! I have found this to be surprisingly common among the less tech-savvy.