If you just 'return' a stored zip file or other file to a browser, you don't have to do anything. For example, if a user visits:

http://www.example.com/example.zip

The zip file will ordinarily be handled by the browser's MIME configuration, and the user will be allowed to download it.

However. If your server's MIME handler configuration is messed up, then the server might try to process the file internally, or send the file as if it were page content. You'd have to talk to your hosting provider or look in your server software manual and tell the server NOT try to try to handle zip files, but send them with a valid MIME type.

If you're 'creating' zip data and sending it back from a request, you would need to specify a Content-Type header that described the content =P