Sounds like a compression issue.  I've had the same trouble before excluding my page.

Are you using PageBlaster or another form of compression?  If you are using PageBlaster have you removed the DNN native compression entry in your web.config and excluded the page by Tabid and page name ( See below )?

If you are not using a 3rd party compression tool you should still exclude it from the native dnn compression engine.  The whitespace filter is what reduced my 4MB+ file to 1K, I'd recommend unchecking the option.

Hope this helps.

Example PageBlaster Excludes

For example <path pattern="56" />

<path pattern="Download.aspx" excludeFrom="All"/>

<path pattern="MakeThumbnail\.aspx|LinkClick\.aspx|\.axd|\.asxh|FileManager|Cart|Install|Downloads" excludeFrom="All" />

Do you use Oracle?  Then try GoldFish.Net

Can you please check your host settings and make sure that 'whitespace' filter is turned off ( not checked )

Something very important:

On my test site (*), I have a fresh 4.5.1, I have put a repository with a small file, and a document module with a link to this file.
The download with the document module is ok, the download with the repository is not ok.
The download with the file manager is ok, the download with the repository is not ok.

For me, the difference is in the way it is handled: "linkclick.aspx" or "fileticket"
Can this help you ??? 

(*) (Locopon, Steve and John, I sent you the creditential), and other people can ask me

DV