def download_as_pdf(self, target_dir=None, pdf_filename=None, paper_size=PaperSize.A4,
                        paper_orientation=PaperOrientation.PORTRAIT, include_appendices=False):
        """
        Retrieve the PDF of the Activity.

        .. versionadded:: 2.1

        :param target_dir: (optional) directory path name where the store the log.txt to.
        :type target_dir: basestring or None
        :param pdf_filename: (optional) log filename to write the log to, defaults to `log.txt`.
        :type pdf_filename: basestring or None
        :param paper_size: The size of the paper to which the PDF is downloaded:
                               - a4paper (default): A4 paper size
                               - a3paper: A3 paper size
                               - a2paper: A2 paper size
                               - a1paper: A1 paper size
                               - a0paper: A0 paper size
        :type paper_size: basestring (see :class:`enums.PaperSize`)
        :param paper_orientation: The orientation of the paper to which the PDF is downloaded:
                               - portrait (default): portrait orientation
                               - landscape: landscape orientation
        :type paper_size: basestring (see :class:`enums.PaperOrientation`)
        :param include_appendices: True if the PDF should contain appendices, False (default) if otherwise.
        :type include_appendices: bool
        :raises APIError: if the pdf file could not be found.
        :raises OSError: if the file could not be written.
        """
        if not pdf_filename:
            pdf_filename = self.name + '.pdf'
        if not pdf_filename.endswith('.pdf'):
            pdf_filename += '.pdf'

        full_path = os.path.join(target_dir or os.getcwd(), pdf_filename)

        request_params = {
            'papersize': paper_size,
            'orientation': paper_orientation,
            'appendices': include_appendices
        }

        url = self._client._build_url('activity_export', activity_id=self.id)
        response = self._client._request('GET', url, params=request_params)
        if response.status_code != requests.codes.ok:  # pragma: no cover
            raise APIError("Could not download PDF of activity {}".format(self.name))

        # If appendices are included, the request becomes asynchronous

        if include_appendices:
            data = response.json()

            # Download the pdf async
            url = urljoin(self._client.api_root, data['download_url'])

            count = 0

            while count <= ASYNC_TIMEOUT_LIMIT:
                response = self._client._request('GET', url=url)

                if response.status_code == requests.codes.ok:  # pragma: no cover
                    with open(full_path, 'wb') as f:
                        for chunk in response.iter_content(1024):
                            f.write(chunk)
                    return

                count += ASYNC_REFRESH_INTERVAL
                time.sleep(ASYNC_REFRESH_INTERVAL)

            raise APIError("Could not download PDF of activity {} within the time-out limit of {} "
                           "seconds".format(self.name, ASYNC_TIMEOUT_LIMIT))

        with open(full_path, 'wb') as f:
            for chunk in response.iter_content(1024):
                f.write(chunk)