I recommend RTF. It's pretty well defined, very stable (the file format hasn't changed much since the year dot), and it's fairly simple to write out with RTF::Writer
. It's also well supported: not everyone, not even every Windows user, has Word -- IIRC Wordpad on Windows (installed by default), TextEdit on Mac (installed by default), and one of the office-type options on Linux support it pretty well. I'd try avoiding colours if possible (I've had odd results using TextEdit), but I don't know how complicated your documents are.
Kids, you tried your hardest, and you failed miserably. The lesson is: Never try.