def pdf(cls, uuid):
        """Return a PDF of the invoice identified by the UUID

        This is a raw string, which can be written to a file with:
        `
            with open('invoice.pdf', 'w') as invoice_file:
                invoice_file.write(recurly.Invoice.pdf(uuid))
        `

        """
        url = urljoin(base_uri(), cls.member_path % (uuid,))
        pdf_response = cls.http_request(url, headers={'Accept': 'application/pdf'})
        return pdf_response.read()