public function getInvoicePdf($letterId = null, $invoiceNo = null)
    {
        $params = array();
        if ($letterId)
            $params['letter_id'] = $letterId;
        if ($invoiceNo)
            $params['invoice_no'] = $invoiceNo;

        $response = $this->api->get(
            $this->url() . '.pdf',
            $params,
            'application/json'
        );

        return base64_decode($response->data->content);
    }