Microsoft have very kindly offered free versions of it's popular Office suite in the Windows Store on Windows 10.
While highly cut down version they are good enough for many and of course will open all Office files fine. OneNote has always been a free app (and a good one at that, too - recently started using it)
Thank you to @Kat for originally sharing this information. These programs are similar in functionality to the 'Office Starter' editions seem preinstalled on some OEM PCs in previous years.