MS Office was never free. Even if you didn't pay for it directly.
MS has 2 different kinds of versions of Office. They have the 365 version which is a subscription version. You pay for it monthly/yearly but you get the latest versions and cloud storage through their OneDrive.
If you don't want a subscription, you can buy the 2016 edition. You pay for it once and can use it on that computer for as long as you'd like. You don't get the version upgrades and cloud storage, though. But it may be more cost effective in the long run depending on how much you use it.
You could illegally obtain it, but we can't help you with that and it is very likely that you will get some sort of virus or malware. So it really isn't worth it.
If you don't want to pay for it, you can use their online only version at office.com It works for the most part, but isn't meant to be a replacement for the full version.
Alternatively, you can use a free and open source equivalent called LibreOffice. It works pretty well and is mostly compatible with Office files. it isn't perfect, but it is a great alternative.