def pdf_field_type_str(field):
  """Gets a human readable string from a PDF field code, like '/Btn'"""
  if not isinstance(field, tuple) or len(field) < 4 or not isinstance(field[4], str):
    return ''
  else:
    if field[4] == '/Sig':
      return 'Signature'
    elif field[4] == '/Btn':
      return 'Checkbox'
    elif field[4] == '/Tx':
      return 'Text'
    else:
      return ':skull-crossbones:'