def _pdf_guess_version(input_file, search_window=1024):
    """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.
    """

    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 ''