public function downloadAttachment($attachmentIndex)
    {
        $fileInformation = $this->GetAttachment($attachmentIndex);
        //build URI to download attachment
        $strURI = Product::$baseProductUri . '/pdf/' . $this->getFileName() . '/attachments/' . $attachmentIndex . '/download';
        //sign URI
        $signedURI = Utils::sign($strURI);
        $responseStream = Utils::processCommand($signedURI, 'GET', '', '');
        $v_output = Utils::validateOutput($responseStream);
        if ($v_output === '') {
            Utils::saveFile($responseStream, AsposeApp::$outPutLocation . $fileInformation->Name);
            return '';
        } else
            return $v_output;
    }