The earlier replies have correctly pointed you to Microsoft's download
site. It seems to work like any other download site: if you can see
what you want you can download it.
According to the download page, to be able to _convert_ files you need
to install the converter on a system running OS 10.4.8 or later.
But it seems that all you need to be able to _use_ the files produced by
the converter is the Office X 10.1.9 update.
We haven't met any .docx files yet, but I expect to convert them on my
OS 10.4.11 system, and pass the resulting files to my wife for her
Office X under OS 10.3.9.