pipのアップデートを実行する

python -m pip install --upgrade pip

<出力結果>
Requirement already satisfied: pip in c:\users\user\appdata\local\continuum\anaconda3\envs\pdf_toexcel\lib\site-packages (21.2.2)
Collecting pip
  Downloading pip-21.3.1-py3-none-any.whl (1.7 MB)
     |████████████████████████████████| 1.7 MB 2.2 MB/s
Installing collected packages: pip
  Attempting uninstall: pip
    Found existing installation: pip 21.2.2
    Uninstalling pip-21.2.2:
      Successfully uninstalled pip-21.2.2
Successfully installed pip-21.3.1

pip -V

<出力結果>
pip 21.3.1 from C:\Users\user\AppData\Local\Continuum\anaconda3\envs\pdf_toexcel\lib\site-packages\pip (python 3.8)