Get specified letter by letter ID
        :param pixelleter_id: ID of the letter
        :return: PDF-Letter-Unicode-String if successful else None

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