Может что-то пригодится:
1. Для .htaccess
<files *.pdf>
forceType application/octet-stream
Header set Content-Disposition attachment
</files>
2. На php
header('Content-type: application/octet-stream');
header("Content-Disposition: attachment; filename=".$_GET['url']);
$x = fread(fopen($_GET['url'], "rb"), filesize($_GET['url']));
echo $x;