Validate that the pdf_path configuration is set and the referenced
        file exists.

        Exits the program with status 1 if validation fails.

def _validate_pdf_file(self):
        """Validate that the pdf_path configuration is set and the referenced
        file exists.

        Exits the program with status 1 if validation fails.
        """
        if self['pdf_path'] is None:
            self._logger.error('--pdf argument must be set')
            sys.exit(1)
        if not os.path.exists(self['pdf_path']):
            self._logger.error('Cannot find PDF ' + self['pdf_path'])
            sys.exit(1)