Once I upgrade to Catalina will I be able to share MS Word documents with other computers that don't have Catalina?

Not necessarily. Word is often per machine, per user license.

However Word is not the only application that can read Word files.

Office 365 is free on the web version as is the most compatible version for those who do not have a current release of Office licensed.

Google Docs, Apple iWork, LibreOffice, OpenOffice, OnlyOffice all offer Word compatiblity to a lesser extent, but short of the person running the same or newer version of Office on their computer, sharing may fail either due to layout concerns, or other features not found in their version of Word "compatible" software.

The other thing to keep in mind, older versions of Office that are 32 bit are not compatible with Catalina or later, in that the application won't run at all. You may be able to read older documents, and save to older formats, but Office 2011, and 2008 may not work at all.

If you have an old iMac, then upgrading to Catalina will require a 64-bit implementation of Microsoft 365 or Office 2021 for Mac, and these are neither free, nor without requiring a license/software upgrade from your old Mac. The actual MS Office documents are bit-independent and can be opened with MS Office intended for Catalina or later, or any application on any computer that can open MS Office documents.

susanfromglencoe wrote:

I have an old iMac that works fine but if I upgrade to Catalina will I be able to share documents with other computers, like my laptop - that don't have it?

Yes. Documents are not 32 or 64 bit, they are documents. You can share, email, use cloud services, etc. to share a document across devices, or with other Office users.