Presently there is no version of Office available that you can install that will reliably render newer Microsoft Word documents on El Capitan.
High Sierra is the oldest OS that will support a newer Office document clone, called Softmaker:
https://www.softmaker.com/en/
If your Mac is capable of upgrading to High Sierra, I would do so regardless, as it won't affect most software compatibility, and improve some.
Older software may be available for El Capitan, but MIcrosoft has turned off the licensing server for versions older than Big Sur compatible version of Microsoft Office.
This means that unless you maintained all the preference and library files of your older install of Microsoft Office through cloning, or a drive that has not yet perished, your older release of Office may not offer any running capability for read/write and printing.
Some not as reliable rendering may be available through Google Docs on Google's website, iCloud apps on iCloud's website, Office 365 on office.com, LibreOffice, and OpenOffice.