Short answer
On a Mac, Word and Pages can both 'Save as PDF', but if you don't have them open you can convert a .docx to PDF right here in Safari or Chrome — the file is processed on your Mac and never uploaded.
If you have Microsoft Word, File → Save As → PDF gives the highest-fidelity result. This browser tool is the quick, no-app option when you just need a shareable PDF of a .docx and want to keep the file private.
It works the same on any macOS version with a modern browser.
Use this in-browser converter, or open the .docx in the free Pages app and export it as PDF. Both avoid needing Microsoft Word.
No — it's converted locally in your browser.