[mariosipad]

@khoinguyen1407

In ../cops/config_local.php set (if using apache)

    /*
     * Wich header to use when downloading books outside the web directory
     * Possible values are :
     *   X-Accel-Redirect   : For Nginx
     *   X-Sendfile         : For Lightttpd or Apache (with mod_xsendfile)
     *   No value (default) : Let PHP handle the download
     */
    $config['cops_x_accel_redirect'] = "X-Sendfile";

and try again.

In apache2 first enable mod_xsendfile (from terminal: a2enmod x-sendfile)

Hope this works, let us know.