Hm, that could be a lot of different reasons why your download will not work properly.
The documents module take care of some core code to download the files, so that you have no chance to add the workaround in the document module.
BTW: It is not really a good idea to provide downloads of this size via http download, for big files it is much better to use a ftp server.
If you like you can try my module DNNPortal-Download for your big files (the module works as described in the workaround).
regards
HP