What do I download to replace Microsoft office for my iMac?

It is not clear what I download to replace Microsoft office on my iOS for my iMac?
I need to be able open and work in my word documents, powerpoint and excel documents.

Perhaps see this question: Does LibreOffice work with iPad?

You said you wanted to replace it “on iOS” for your “iMac”. Which one are you looking for a replacement for? I assume it’s iOS, because LibreOffice is available on MacOS. Microsoft Word, Powerpoint, and Excel each cost nothing on iOS, so most people just use those. You can also use Google’s apps, such as Docs, Slides, and Sheets. I’m sure there are other options if you look for them as well. Unfortunately, LibreOffice has no iOS support.

Edit: LibreOffice does have one iOS app, LibreOffice Impress Remote, but I don’t think that’s what you’re looking for.