Further to what rickky at gmail dot com was saying, I've had that problem while trying to cache zip files and found that I had to set the permissions of the containing folder to 777 to get it to work.
Because this is a potential security weakness if on a public viewable folder, I'd recommend moving the folder so that it is no longer within public_html. You can then use readfile() to output the archive to the browser, with some HTTP headers to tell the browser it is a zip file.