=== php.net ===
$filename = $_SERVER['DOCUMENT_ROOT'] . "/path/to/file/my_file.pdf";
header("Cache-Control: public");
header("Content-Description: File Transfer");
header('Content-disposition: attachment; filename='.basename($filename));
header("Content-Type: application/pdf");
header("Content-Transfer-Encoding: binary");
header('Content-Length: '. filesize($filename));
readfile($filename);