Hi,
IZWebFileManager provides you ability to browse files and download/open them over internet.
It means that for opening file you have to download it first. Once you downloaded file you get the copy and have to care to upload changes back to server.
MS Office by design allows to edit files directly on server, But it requires server to be configured to support this, and possible the way to open such files with MS Office is different to just download it by url as IZWebFileManager does.
In other words you can't modify files directly on server by default, but theoretically it is possible with additional effort.