If you download and install the new Windows 10 for phones preview build 10051, there's one known issue that we felt that should be given a special mention. You won't be able to open any Office files if you decide to try out this build on your supported Lumia smartphone:
Microsoft put this rather large problem in its long list of known issues for the 10051 build. Here are the specifics:
"The Office Hub has been removed from this build. You won't be able to open Word, Excel, PowerPoint or OneNote files in this build. Preview versions of the Universal Office apps will be available in the coming weeks."
So if you use your Lumia smartphone a lot to view Office files, we would strongly suggest bypassing this Windows 10 for phones preview release. As the known issues list states, Microsoft is developing new versions of its Office apps that will work universally across all Windows 10 devices.