Originally Posted by Veridical Fiction

You won't need a DI box if you have 1/4" line inputs on the PA mixer. (I'm pretty sure you do)

The output from your keyboard's headphone jack is line level, so simply acquire a stereo audio cable like this:
and a 1/4" adapter like this:


and plug your keyboard's headphone out into a channel's line input on the PA mixer.
The amp driving the PA mixer will make sure your audio signal is properly amplified along with whatever else you have plugged in.

Adjust the tone shaping controls on your channel to taste.