Dude there's LibreOffice, Google Docs, Apple's Pages app, maybe some others. Why don't you try each of them and see how you like them?
LibreOffice is outstanding. I've used it since 1999 when it was Star Office. You can write anything and export to PDF easily. You can also share your work with MS Office users. Writing business letters is a simple discipline. Content and simplicity go hand in hand. LibreOffice will also work for you on Windows or Linux. Finally anyone who claims LibreOffice is dated probably doesn't know that humans have been writing for 4 thousand years and we have never had it so good as we do now!
My experience with Libreoffice is a bit dated (2 years), so take with a pinch of salt. If you don't need to work on the document with MS word (or do not need to exchange it with somebody using MS word) then Libreoffice Writer is more than fine. It will do whatever MSW does, with equal quality (I found that formatting was getting screwed when passing documents between the two). Also, because I use MSW so often at work, I got used to the ribbon menu, and LO menu looks outdated (to me). Apple's Pages is all you need if you are on Mac (and I have found it to be more compatible than LO with MS Word). Conversely, if you need to edit the document with other people (e.g. some school group work) Google Docs is hard to beat. Edit: typos.
Definitely give Libre office a chance it's a great free alternative office suite. When we have departments that aren't willing to fork out the money for MS Office we deploy Libre office to their systems often without anyone ever realizing they're not using Microsoft. These are non tech people too...
If you're after 'The Most Compatible Free Office Suite', in terms of file compatibility, user interface/experience, etc, you can't go past WPS Office (wps.com) for Windows or Linux.
If you can stomach learning to use it, although it's much simpler (IMO) at scale!, LaTeX is very nice. https://www.sharelatex.com/ is even nicer.
Without knowing what you use it for it's impossible to say.
If it's just general word processing there are a few that are very good.
Libre Office, Google Docs and AbiWord are fine.
wps office, despite they're also offer the premium.