I thought it would be a more interesting invention, but printing press (burner) is good enough.
Zero hostility here, but I'm gonna be a nerd here and say the printing press would be an amazing invention, it changed our world, literally. We just have been used to it.
That said, a form of printing across vast distances is way more than a printing press. It's more like having snapchat or something. Imagine being able to instant message your forces in the medieval era (and even with magic, this allows laymen to do so, excellent for levied troops). The strategic advantage would be unfathomable. Better yet, the message 'self destructs' by fading away.