def handle_starttag(self, tag, attrs):
    '''
    PDF link handler; never gets explicitly called by user
    '''
    if tag == 'a' and ( ('class', 'download-pdf') in attrs or ('id', 'download-pdf') in attrs ):
        for attr in attrs:
          if attr[0] == 'href':
            self.download_link = 'http://www.nature.com' + attr[1]
PDF link handler; never gets explicitly called by user