Geoff,
Can you provide a sample file that shows this problem? I suspect that the file is either empty, or otherwise corrupt. Does it happen once when you try to download the same document 10 times?
When opening PDF files from a web server, there are a number of things that can go wrong: The server can send out a corrupt file, so what you are receiving is already broken, and nothing your system does can fix that.
It could be the browser that messes up the download, it could be the operating system, it could be the PDF viewer, ... However, you mentioned that you've tried different browsers and different PDF viewers (and I assume you had the problems with all configurations, at least you don't say that some configurations did work successfully.
So, having access to a sample file could potentially help to understand what's going on.
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com