Yo uso algo asi:
Código PHP:
<?php
$fullpath = '/ruta/a/tu/archivo.ai';
header("Pragma: public");
header("Expires: 0");
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("Cache-Control: private", false);
header("Content-Transfer-Encoding: binary");
header("Content-Type: application/octet-stream");
header("Content-Length: " . filesize($fullpath));
header("Content-Disposition: attachment; filename=" . basename($fullpath));
readfile($fullpath);
exit();
?>