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.

[Guessed]

The problem is caused by the Mac user typing the filename including what he thinks the right suffix is. Macs don't need a file suffix to work out what file type they are dealing with. Instead they have another 'invisible' file called a resource fork which includes the file type information.

The File Save format is somewhere obvious in the save as dialog but the user has to actually look at it. It is much the same as when saving a Word file in Windows so the different OS is not a hindrance to your being able to show him where he is going wrong.

[Gweilo]

Originally Posted by Guessed

....

Thanks, that's basically what I guessed was going on, but I didn't want to front at his office without some confirmation. Trying to walk him through it via email or phone would be hopeless without knowing exactly what the screen shows. Hopefully I'll be able to find it in person.

-- Is there a standard way on Macs to see what the resource fork says a file is?
Or does it need some 3rd party utility?

[Guessed]

You can see the relevant Mac dialogs in these videos

What the save dialog looks like in Mac Word
https://www.youtube.com/watch?v=vCjeP4v2V5Y

Method of saving to PDF from Mac Word
https://www.youtube.com/watch?v=b3nPtXIaYNU

I don't know how to view a Resource Fork file on a mac. In Windows (for example on a USB thumbdrive taken from a mac), I can view a resource fork by looking for a file with the same name as the actual file but with '._' added to the start of the name. I then open that file using Notepad to see the contents which are largely rubbish but you can see in cleartext the operating system and file type.