Haber intenta con eso :
Código PHP:
<?php
ob_start();
header("Content-type: application/pdf");
header("Content-Length: ".(string)(filesize($str_file_contents)) );
header("Content-Disposition: attachment; filename=preview.pdf");
header("Cache-control: private");
header("Pragma: public");
header("Content-Transfer-Encoding: Binary")
print($str_file_contents);
ob_end_flush();
?>