Text exceeding X characters will be automatically sent as a .txt file to your contact. This does not require your contact to have any third-party programs installed, as Notepad is already available in Windows. This has been implemented by Messenger Plus! and has nothing to do with how Messenger's protocol works on the server.
Moreover, Microsoft Word is used by millions of users, so don't you worry, the majority has it, and they're not stupid/ignorant enough, for them not to know how to use it.