WC_Download_Handler::init PHP Method

    public static function init()
    {
        if (isset($_GET['download_file']) && isset($_GET['order']) && isset($_GET['email'])) {
            add_action('init', array(__CLASS__, 'download_product'));
        }
        add_action('woocommerce_download_file_redirect', array(__CLASS__, 'download_file_redirect'), 10, 2);
        add_action('woocommerce_download_file_xsendfile', array(__CLASS__, 'download_file_xsendfile'), 10, 2);
        add_action('woocommerce_download_file_force', array(__CLASS__, 'download_file_force'), 10, 2);
    }

     *
     * IE bug prevents download via SSL when Cache Control and Pragma no-cache headers set.
     *
     * @param array $headers
     * @return array
     */
    public static function ie_nocache_headers_fix($headers)
    {
        if (is_ssl() && !empty($GLOBALS['is_IE'])) {
            $headers['Cache-Control'] = 'private';
            unset($headers['Pragma']);
        }
        return $headers;
    }
    /**
     * Die with an error message if the download fails.
     * @param  string $message
     * @param  string  $title
     * @param  integer $status
     * @access private
     */
    private static function download_error($message, $title = '', $status = 404)
    {
        if (!strstr($message, '<a ')) {
            $message .= ' <a href="' . esc_url(wc_get_page_permalink('shop')) . '" class="wc-forward">' . __('Go to shop', 'woocommerce') . '</a>';
        }
        wp_die($message, $title, array('response' => $status));
    }
}
WC_Download_Handler::init();