A secretary in a office gives you a rm to fill out. When you finish it, you say: "I finished".
A secretary in a office gives you a form to fill out. When you finish it, you say: "I finished". 

Are there others ways (both formal and casual) to say the same?

Hi Victor!

You could say: "I'm finished," or "I'm done," or informally just "Done!".

I hope this helps - happy studying!
Matt