The text viewer plugin isn't really intended to be used outside of the viewer pane, and doesn't normally get used in the standalone viewer window at all. I don't think there is a way to do what you want with that. There might be other text plugins out there which do that though.
Other document types, like Word and PDF, will come down to what their respective viewers do, which we don't have a lot of control over. (There's no PDF or Word viewer code inside of Opus; we just use the preview handlers installed on your system for those document types.)