public function convertToText()
    {
        $pathInfo = pathinfo($this->filename);
        switch (strtolower($pathInfo['extension'])) {
            case 'docx':
                return $this->readDocx();
            case 'xlsx':
                return $this->readXlsx();
            case 'pptx':
                return $this->readPptx();

            default:
                throw new \InvalidArgumentException('File extension "' . $pathInfo['extension'] . '" not supported!');
        }
    }