Looks for all the world like a permissions problem. If you have no control over the servers then you probably can't change the access permissions for the directory you are storing the files in either.
If you are sure that the permissions aren't the issue, you might try not just linking directly to the file but rather using readfile() to send it.
From http://php.net/manual/en/function.readfile.php
<?php
$file = 'monkey.gif';
if (file_exists($file)) {
header('Content-Description: File Transfer');
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename='.basename($file));
header('Content-Transfer-Encoding: binary');
header('Expires: 0');
header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
header('Pragma: public');
header('Content-Length: ' . filesize($file));
ob_clean();
flush();
readfile($file);
exit;
}
?>
