I'm not sure, but there may be a way of forcing it using a combination of content types and Disposition headers...
Essentially you have no control over the way content is handled at the client side, since it gets managed in 'the standard way' for that content type. eg The default for a Word
doc is to open it using the plug-in for the browser. It could just as easily be to play the song, display the movie, etc etc.
Usually a browser will use the content-type returned by the server to decide what to do with the content. If you say the content is 'text/html' it will be displayed differently to 'text/xml' or 'text/plain'.
The problem with downloading files is that if a content type is not specified, the browser can use the file extension to determine the content type.
A way of hacking this could be to return the content without specifying the content type (or defining some other type of content?) then use the disposition header setting to define the filename. I doubt it would work tho.
Your best bet is to zip the docs or tell users to "right click and download"
Dave