Give it time
Microsoft's document formats **have to** be kept a secret. This is because, unlike physical objects (which may have parts such as blades which go blunt, rubber belts or idlers which perish, heat exchangers which corrode or metal parts which scrape against metal parts), software **doesn't wear out**.
The only way that Microsoft can build in planned obsolescence into Office is to release a new version with exactly the same functionality but an incompatible file format, stop making the old version and give away (or rely on the pirates to give away) a few complimentary copies of the new version. Then, when all your friends are sending you documents in the "new" format, you will find it easier to upgrade your own copy of Office than to explain patiently to everyone that they need to go into settings -> advanced -> file format -> default, select "Save as .doc : Word 2000 / XP", and then ignore the requester which pops up everytime they save a file saying that some features may not be supported when saving in this format and wouldn't they rather save in the latest and greatest new file format instead?
Of course, having bought (we'll assume you're a business and therefore unable to get away with using pirated software, though some of the people with whom you are corresponding clearly are) the latest brand-new version of Office, you will now find that your computer slows to a crawl and so be forced to upgrade your motherboard. And remember, all this is for no noticeable benefit for you. The **only** thing that New Office does that your Old Office didn't do is save its files in a different format. And consume more CPU cycles while doing it, of course .....
If someone were able to write a program that could read "new format" files and spit out "old format" files, then you would be able to exchange documents with your friends just fine, and so never need to upgrade. Hence, documenting file formats would be professional suicide for Microsoft. So they aren't going to do it.
(How it's done on the Other Side of the Divide is this. The best and most complete documentation for any file format is always the Source Code to the program out of which it was saved. Knowing that people will be looking at your Source Code, you already made your file format as extensible as could be and wrote your file-reading code as bomb-proof as could be, just so nobody would have anything to point and laugh at; so when loading a "new" file into the "old" program, new features probably will just get ignored. There is generally no good reason to change a file format radically, unless you introduce a feature which is so new and so radical that it really cannot be represented at all in the old format.
In situations where code is thoroughly audited, "shiny and new" isn't considered as important as "proven stable and secure" and the latest versions of everything are not necessarily available immediately -- Debian, for instance -- a simple conversion tool can be created by splicing the new "read" routine onto the old "write" routine.)
There are two ways out of this situation, and it's a matter of time before one of them becomes reality:
Sooner or later, someone high-profile will end up in the situation where a vitally-important Word document, created with a version of Office which is no longer obtainable or will only run on hardware which no longer exists, is no longer accessible. Then there's going to be an interesting show.
Or it's possible that a decompiler will be invented before then; in which case, there won't be such a thing as closed source anymore. But betting the future accessibility of your old documents on such an outcome still too big a gamble for most organisations.