Git is agnostic in the sense that it doesn’t matter which files you put under version control.
When git can’t recognise the type of a file it just treats it as binary data for versioning purposes; so diff etc. will just state that the files are different by a number of bytes.
So to answer your question: Yes, Git can be used to manage versions of word documents etc., furthermore using gitattributes you can even diff word documents although I’m not sure if this is possible for xls files.
To learn more about the possibilities which gitattributes provide and to see some examples for diffing word files you can read the Customizing Git – Git Attributes chapter from the Gitpro book.