e.g. you're using php and $filepath contains pdf file path.
so you can write javascript like to to emulate download dialog box.
<script language="javascript">
window.location.href = '<?php echo $filepath; ?>';
</script
Above code sends browser to pdf file by its url "http://www.domain.com/files/filename.pdf". So at last, browser will show download dialog box to where to save this file on your machine.