Try making a copy in File Manager, can you open the copy?

Similarly, what happens if you rename the doc file?

Do you know if the doc has external links in it, eg hyperlinks to other files or internet? Just wondering if Word is trying to access something which doesn't exist anymore.

Is the doc in a 'normal' folder? Ie not on a server or other machine on a network, or protected folder on your machine, like Program Files etc.

Can you open it in Notepad? [just to establish if it can open at all]

It might be corrupted, can the source provide a different copy for download?
If it's a general download, are other users having a problem?