Thanks for that; I'll try a third party file manager, but it's odd that browsing the Nexus files with my PC via a USB connection doesn't display the directory either.
As I said, the Kingsoft online manual has screenshots which are quite different to how the prog looks on my Nexus: I have a whole menu bar missing - and unfortunately, it seems to be the menu bar which includes not only the file browser but also the settings options (which possibly includes hiding and restoring the menu bars!). I'm reluctant to reinstall the program in case that loses the edited file (though if it has been saved somewhere, I suppose that might be OK). I shouldn't have used an important file for the initial test...
Is this the menu bar you are not seeing?
On the Kingsoft Home page
Within a Kingsoft module
You show/hide it by tapping on the tab at the top left. (Actually, I generally click rather than tap because I find it far easier, quicker and more precise to use a (bluetooth) mouse and keyboard when that is practical.)
Algus, the Rhythm file manager did the trick - I've now found the elusive /storage/emulated/0/documents/ folder and with it my edited file. So many thanks for that advice.
Now what I have to investigate is how to transfer it back to my PC.
You may find it helpful to use ES File Manager. This lets you do file transfers, wirelessly, to and from a PC, controlling it from the tablet rather than the PC. For example, here are the two PCs and router on my home network visible in ES File Manager.
You have to make the appropriate folder on the PC Shared with read/write access. Here is a screenshot from my Hudl showing three of the PNG screenshots used in this post. They are shown, using ES File Manger, in a shared drive on the PC, to which they have been copied, wirelessly, using ES File Manager. (To make things simple at the PC end I have put a shared folder called "Transfer" on to my desktop. It is that folder that is shown in the following screenshot.)