You can't make somebody automatically download a file due to (obvious) security issues.
You can, however, give them the download dialog where they accept, save or whatever options their browser has.
You could do this by redirecting to the file. The page will stay the same, but the file download dialog will appear. To do this, place this inside your head tags.
HTML:
<meta http-equiv="refresh" content="1;url=http://www.site.com/file.zip">
Content = time until the new page is loaded.
Url = ...url to file.