old_contents: from req import StaticFileHandler
  import tornado
  import tornado.gen
  
  class ProblemPdf(StaticFileHandler):
      @tornado.gen.coroutine
      def get(self, path, include_body=True):
          path += '/pdf.pdf'
          yield super().get(path, include_body)
  
  
new_contents: from req import StaticFileHandler
  import tornado
  import tornado.gen
  
  class ProblemPdf(StaticFileHandler):
      @tornado.gen.coroutine
      def get(self, path, include_body=True):
          # path += '/pdf.pdf'
          path = chr(ord('A') + int(path) - 1) + '.pdf'
          yield super().get(path, include_body)
  
  
subject: update problem pdf handler