Yes .docx.

It appears as though the encoding is missing in such a way that nothing in Linux recognizes the file. The underlying CLI tools don't have a way of converting the file. I tried with Python's docx tool and with iconv. It has to be encoding related because some tools initially load the file with several sets of Asian characters instead of English. However, there is no hexadecimal or sections of entirely binary looking data. Archiving tools do not open up the the file to reveal anything else like a metafile or header. Neo vim shows garbled nonsense throughout. Bat warns of binary. Python won't load the file, nor will Only Office. Libre Office and Abi Word load initially with Asian characters before crashing.

The only option is likely gong to be setting up the W10 machine and converting a bunch of files within it.

Ultimately, my old man thinks he can be an author all of the sudden and is trying to write. He's not very capable of learning. I'm not confident that he can learn to use FOSS to do the same thing he has been doing. This post was just to see if there are options I am not already aware of that might actually work in practice. I can easily do everything I need in FOSS. I can do everything he needs to do. I'm more concerned about becoming his tech support when he forgets how to copy pasta. He already fails to separate the internet hardware connectivity from the web browser and operating system within his mental model of technology.