I have a signed document in Odoo 14 sign module. When I click download, Odoo prompt me Internal Server error with the following error code. The signed document is 28Mb. 

Odoo Server Error

Traceback (most recent call last):
  File "D:\Program Files\Odoo 14e 18122021\server\odoo\addons\base\models\ir_http.py", line 237, in _dispatch
    result = request.dispatch()
  File "D:\Program Files\Odoo 14e 18122021\server\odoo\http.py", line 684, in dispatch
    result = self._call_function(**self.params)
  File "D:\Program Files\Odoo 14e 18122021\server\odoo\http.py", line 360, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "D:\Program Files\Odoo 14e 18122021\server\odoo\service\model.py", line 94, in wrapper
    return f(dbname, *args, **kwargs)
  File "D:\Program Files\Odoo 14e 18122021\server\odoo\http.py", line 348, in checked_call
    result = self.endpoint(*a, **kw)
  File "D:\Program Files\Odoo 14e 18122021\server\odoo\http.py", line 913, in __call__
    return self.method(*args, **kw)
  File "D:\Program Files\Odoo 14e 18122021\server\odoo\http.py", line 532, in response_wrap
    response = f(*args, **kw)
  File "D:\Program Files\Odoo 14e 18122021\server\odoo\addons\web\controllers\main.py", line 1393, in call_button
    action = self._call_kw(model, method, args, kwargs)
  File "D:\Program Files\Odoo 14e 18122021\server\odoo\addons\web\controllers\main.py", line 1381, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "D:\Program Files\Odoo 14e 18122021\server\odoo\api.py", line 399, in call_kw
    result = _call_kw_multi(method, model, args, kwargs)
  File "D:\Program Files\Odoo 14e 18122021\server\odoo\api.py", line 386, in _call_kw_multi
    result = method(recs, *args, **kwargs)
  File "D:\Program Files\Odoo 14e 18122021\server\odoo\addons\sign\models\sign_request.py", line 167, in get_completed_document
    self.generate_completed_document()
  File "D:\Program Files\Odoo 14e 18122021\server\odoo\addons\sign\models\sign_request.py", line 513, in generate_completed_document
    can.drawImage(image_reader, width*item.posX, height*(1-item.posY-item.height), width*item.width, height*item.height, 'auto', True)
  File "D:\Program Files\Odoo 14e 18122021\python\lib\site-packages\reportlab\pdfgen\canvas.py", line 925, in drawImage
    rawdata = image.getRGBData()
  File "D:\Program Files\Odoo 14e 18122021\python\lib\site-packages\reportlab\lib\utils.py", line 902, in getRGBData
    annotateException('\nidentity=%s'%self.identity())
  File "D:\Program Files\Odoo 14e 18122021\python\lib\site-packages\reportlab\lib\utils.py", line 1406, in annotateException
    rl_reraise(t,v,b)
  File "D:\Program Files\Odoo 14e 18122021\python\lib\site-packages\reportlab\lib\utils.py", line 144, in rl_reraise
    raise v
  File "D:\Program Files\Odoo 14e 18122021\python\lib\site-packages\reportlab\lib\utils.py", line 886, in getRGBData
    if Image.VERSION.startswith('1.1.7'): im.load()
Exception

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "D:\Program Files\Odoo 14e 18122021\server\odoo\http.py", line 640, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "D:\Program Files\Odoo 14e 18122021\server\odoo\http.py", line 316, in _handle_exception
    raise exception.with_traceback(None) from new_cause
AttributeError: module 'PIL.Image' has no attribute 'VERSION'
identity=[ImageReader@0x272efeee448]