code: def _pdf_guess_version(input_file, search_window=1024):
      
  
      with open(input_file, 'rb') as f:
          signature = f.read(search_window)
      m = re.search(br'%PDF-(\d\.\d)', signature)
      if m:
          return m.group(1)
      return ''
docs: Try to find version signature at start of file.
  
      Not robust enough to deal with appended files.
  
      Returns empty string if not found, indicating file is probably not PDF.