Definitely not a supported setup.
From the KB article:
"Adobe Technical Support does not recommend or support having multiple versions of Acrobat or Adobe Reader installed on the same machine (for example Acrobat 8, with Adobe Reader 9 or Acrobat 7 with Adobe Reader 8). Acrobat and Adobe Reader have numerous components that have similar functionality and can cause conflicts on a machine. The most prevalent among these conflicts involves the plug-ins for rendering PDF files in the browser,"
It is strange though, why would other pdf's open fine from other sites?
Could be a
n issue with the versions of the PDF's on your site (just a guess here). If they are newer than 6 and 6 is trying to open them, 6 could be mistakenly thinking that they are corrupt.
Hmm...Strange thing is the files are showing 0kb right at the desktop, just not sure if Acrobat or AR do anything to decode while it is being downloaded. If so, then I could see an issue with 6 not being able to decode upon download, since we use the newer Acrobat 9.