Word Mac saving as PDF not docx

[Gweilo]

A guy using MS Office on Mac is trying to send me (on Win) some Word files.
The files have names like "file.docx", but in fact are PDFs.

I asked him to change the format, but all he actually does is change the filename.
I can go to his office and try to work it out, but he's using Word on Mac, not sure of version, and I have no experience with that.

Why is it doing this?
Will the file save format be somewhere obvious?
And how on Mac can you tell the real file format?


On PC I just look inside the file with the viewer in Far and see the PDF header.

I don't need a work-around, I want to prevent the problem. I know I can convert the PDF back to text, more or less, but it's not an exact process, paragraphing etc. is not preserved, and we're going to be transferring dozens of files and I need it to be error free.