docs: def get_letter_as_pdf(self, pixelletter_id):
          """
          Get specified letter by letter ID
          :param pixelleter_id: ID of the letter
          :return: PDF-Letter-Unicode-String if successful else None
          """
          pdf_letter = self._make_get_request('letters/{}.pdf'.format(pixelletter_id))
  
          if pdf_letter:
              return pdf_letter
  
          return None