hook_file_download is not called

I have implementation of hook_file_download for accessing private files but it is not called

I have this code

<?php
function mymodule_file_download($uri) {
    watchdog('mymodule', 'hook file_download was called');
    $file = db_query("SELECT * FROM {file_managed} WHERE (uri = :uri)", array(':uri' => $uri))->fetchObject();
    /*$headers = array(
        'Content-Type: application/pdf',
        'Cache-Control' => 'private',
        'Content-Length' => $file->filesize,
        'Last-Modified' => $file->timestamp,
        'Content-disposition: attachment'
    );*/
    $headers = file_get_content_headers($file);
    return $headers;
}
?>

I have private file in my private folder /sites/default/private_files/myfolder/test.pdf
When I insert this URL in browser I got message 403 forbidden.
I have set private_files as private in administration what generates this .htaccess file:

SetHandler Drupal_Security_Do_Not_Remove_See_SA_2006_006
Deny from all
Options None
Options +FollowSymLinks

I have also tried to comment the two Options lines

watchdog message is not saved, so I think the hook is not called.

Can anybody help what am I doing wrong?