many S60 devices include in their description that they are compatible with office files. As far as I know, those files can only be opened when downloading extra software like "quickoffice", which is NOT always present in the device.
For instance, I just bought a 5800 which came without quickoffice. Its description page http://www.forum.nokia.com/devices/5800_XpressMusic
in the "Messaging" group says "Document Formats - Excel, PDF, Powerpoint, Word, Zip" which is not accurate.