Here's how you can use LibreOffice running in headless mode with Python to convert a docx file to a PDF:

import sys
import subprocess
import re


def convert_to(folder, source, timeout=None):
    args = [libreoffice_exec(), '--headless', '--convert-to', 'pdf', '--outdir', folder, source]

    process = subprocess.run(args, stdout=subprocess.PIPE, stderr=subprocess.PIPE, timeout=timeout)
    filename = re.search('-> (.*?) using filter', process.stdout.decode())

    return filename.group(1)


def libreoffice_exec():
    # TODO: Provide support for more platforms
    if sys.platform == 'darwin':
        return '/Applications/LibreOffice.app/Contents/MacOS/soffice'
    return 'libreoffice'

More info can be found in this article.

If you prefer to have an API version running in a Docker container, you can go for Gotenberg.