Is there a way to download Microsoft Office for free on iMac?

No -- there are no free versions of Microsoft Office that you can download.

You can use the online web versions for free. See the following: https://www.microsoft.com/en-us/microsoft-365/free-office-online-for-the-web

Another free option is LibreOffice. It is similar to and quite compatible with Microsoft Office. See: https://www.libreoffice.org/

You don't need Microsoft Office, especially if you are averse to paying Microsoft's ever-increasing fees for the privilege of using their bloatware.

Apple's Pages, Numbers, Keynote are all free, as are other free or nearly free Microsoft equivalents.

‎Pages on the Mac App Store

‎Numbers on the Mac App Store

‎Keynote on the Mac App Store