public function setDocumentProperty($propertyName = '', $propertyValue = '')
    {

        if ($propertyName == '')
            throw new Exception('Property name not specified');

        //build URI to replace image
        $strURI = Product::$baseProductUri . '/pdf/' . $this->getFileName() . '/documentProperties/' . $propertyName;

        $putArray['Value'] = $propertyValue;
        $json = json_encode($putArray);

        //sign URI
        $signedURI = Utils::sign($strURI);
        $responseStream = Utils::processCommand($signedURI, 'PUT', 'json', $json);

        $response_arr = json_decode($responseStream);

        return $response_arr->DocumentProperty;
    }