Whether you've received a Microsoft Office file by email or perhaps copied it onto your communicator using PC Suite, it's often best to work using Symbian OS's native Word and Sheet formats rather than having to wait every single time you open, save (e.g. when switching away to another app) or close an Office file. To save a file in native format, use the 'epoc/app' option in the 'Save as' dialog.
And, of course, if you're creating a new document, start in native (EPOC) format and then it'll be converted when dragging and dropping back to your PC (or you can re-save it manually).