Put this in the HTML head section, setting the url
var to the URL of the file to be downloaded:
<script type="text/javascript">
function startDownload()
{
var url='http://server/folder/file.ext';
window.open(url, 'Download');
}
</script>
Then put this in the body, which will start the download automatically after 5 seconds:
<script type="text/javascript">
setTimeout('startDownload()', 5000); //starts download after 5 seconds
</script>
(From here.)