I tried the command and it is working for me:
C:\Users>workon python37
(python37) C:\Users>pip install Pillow==5.3.0
Collecting Pillow==5.3.0
Downloading https://files.pythonhosted.org/packages/55/ea/305f61258278790706e69f01c53e107b0830ea5a4a69aa1f2c11fe605ed3/Pillow-5.3.0-cp37-cp37m-win_amd64.whl (1.6MB)
100% |████████████████████████████████| 1.6MB 3.9MB/s
Installing collected packages: Pillow
Successfully installed Pillow-5.3.0
You are using pip version 18.0, however, version 18.1 is available.
You should consider upgrading via the 'python -m pip install --upgrade pip' command.
You should check if pip is installed in your python3.7. It might be not even installed.
In your tags, you have mention Pycharm and in it, you are not installing packages with pip it has its own packages ocntrol ui.
Pycharm package install guide