header("Pragma: public");
header("Expires: 0");
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("Content-Type: application/force-download");
header("Content-Type: application/octet-stream");
header("Content-Type: text/plain");
header('Content-Disposition: attachment; filename="'.$arquivo.'"');
header("Content-Transfer-Encoding: binary");
header("Content-Length: ".$size);
echo $conteudo;
exit;
e funciona perfeitamente :)