A CAD file is not an image. It is the instructions from the app to design something in either 2D or 3D and thus may only be able to be usefully opened by the original program that created it. Even if the file is opened and it contains text it might make no sense to anyone reading it or any other program since it likely has a unique and maybe proprietary structure from the creating app.

When I use freeCAD I can export the designed object in a language that can be read by a slicer in preparing the code to send to a printer, but the original file is AFAIK not usable by any other 3D CAD program.

I'm giving up. Will try to find a Windows computer to borrow or buy cheaply. Thanks to all who replied.

You can always install Windows into a virtual machine and then delete it if you don't need it any more.