Microsoft is going to put their Office desktop apps in the Store in the near future. As of this writing, you can find app links in the Store using search, but they do not work. This should resolve over the next few months running up to the release of Windows 10 Fall Creators Update (Redstone 3).
Availability of Microsoft's Office suite in the Windows Store is a very significant event, especially for Windows 10 S. Windows 10 S is the new edition for Windows 10 with the big difference from regular editions being that it does not have Win32-app support: you can only install apps that are available in the Windows Store. Microsoft says this change allows it to greatly improve and enhance the security of the OS. Win32 apps converted using Project Centennial (Desktop app converter tool) may run on Windows 10 Cloud though. It is not possible to download and install apps from outside of the Windows Store.
Microsoft didn’t reveal when exactly the desktop Office apps will be available on the Store. However, most of the Office desktop apps have already shown up in the Windows Store search results, but you can’t actually install them yet.