In reply to pjcollinson:
The 'explanation' is that the Save function in the Apple document handling applications is basically redundant. They save the latest version of the original document every time you close the document window or quit the app (which is
kind of handy if you are the sort of dimwit who is always forgetting to save) or when you do a Save (which makes sense) but they also do it when you do a Save As. I
think the idea is that you don't have to remember to do a Save As when you want to 'fork' your document - hence why the function is 'hidden' on the File menu - because the application keeps a history of all your versions of it.
This background saving function uses a variation on the Time Machine functionality. AFAIK you don't actually have to have Time Machine enabled for them to do it, which is just as well because otherwise a "fix" might be to turn Time Machine off, which would be flippin' daft.
You can get back to the previous version of the original document via the Revert To function lower down the File menu. It offers the previous version to the one that's on the screen at the time, or you can choose the Browse All Versions... option to get a Time Machine-like display of all the previous versions it has saved, and pick the version you want from that.
As well as Preview and Pages, TextEdit also works this way.
I think it's one of those 'features' which careless users, or users who can't be arsed to understand how files and directories actually work, find useful. It might also be something to do with MacOS/iOS convergence, ie making a PC behave more like a tablet, which is obviously a
brilliant idea*. (The fact that, IIRC, the Google Docs apps - which are web-based apps with nothing in common with the Apple applications in terms of underlying code - work in a similar way is I think indicative of the same thing.)
As others have said, the "fix" is to use a non-Apple application like Word, or one of the free, open source office suites that can play with Microsoft document formats, like Libreoffice.
* In case you were wondering: I too find this functionality confusing and not nearly as helpful as it is no doubt supposed to be. I've spent forty-odd years managing files and folders, way back to when I was a coder on PDP-11s. I don't want the computer to do it for me, and when it comes down to it I don't trust the computer to do it for me!
