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