Thanks Chris for your great reply!
I see the main code I need is window.location.href="[some_file_here]"; in the drop area.
Now in
http://www.dhtmlgoodies.com/scripts/dra ... rop-1.html, I can change <div class="dragableBox" id="box1">download_file.zip</div>
Add window.location.href=document.getElementById(idOfDraggedItem).innerHTML;
in the dropItems() to initiate the file download.
Thank you very much!