Judging by experience, 'using Odoo on Windows' usually means the setup was done by downloading the odoo_18.latest.exe (or an earlier version) from https://odoo.com/page/download. If this isn't the case for you, based on the problem you're describing, this is probably still applicable (your actual paths then may differ however).

In short 'python' most likely doesn't mean the python you need.

Just 'python' in CLI means the python defined in your system's Environment Variables. In your case, this does not seem to be the interpreter your Odoo setup is using.

By default, when installing Odoo using the .exe, a dedicated python interpreter is along with it. This installation of python, by default, is located in 

C:\Programs (x86)\Odoo 18.0.YYYYMMDD\python\python.exe

This means, you will need to run

"C:\Program Files\Odoo 18.0.YYYYMMDD\python\python.exe" -m pip install fillpdf

in order to install fillpdf in the correct location and for the correct python interpreter (just ensure you replace the 'Odoo 18.0.YYYYMMDD' with your actual version and build date as given in your explorer).

Since this python presumably isn't defined in your PATH vars, as described above, the installation may print some warnings, which in your case, are probably not relevant at this point. If you're still facing issues, this output however could be helpful for further debugging.