You probably need to use a format (such as RTF, Tex, etc) that specify fonts, unless you want to make your own format. AFAIR, Java doesn't provide document writing libraries, so you'll have to ...

In this post, I introduce an open source library project for reading and writing Microsoft Excel spreadsheets.

I read that java applets aren't allowed to read or write to files fullstop but i can certainly read from a file so I thought my understanding of java applet security might need a little updating ...