You can connect your PSR-S770 to your iPad using Apple's Camera Connection Kit. (Yamaha also makes a connector, but it costs more than Apple's Camera Connection Kit.) I do this with my PSR-E/YPT models from time to time.
Or, if you get a Bluetooth MIDI adapter for your PSR-S770, you can connect to the iPad via a wireless Bluetooth connection. Yamaha makes two of these, a USB Bluetooth adapter and a MIDI Bluetooth adapter. The USB version has to be plugged into a power socket, then connected to the keyboard's USB To Host port using a standard USB cable. In contrast, the MIDI version looks like a short MIDI cable and you just plug each end into one of the keyboard's MIDI ports (i.e., the cable goes from MIDI Out to MIDI In). I imagine the MIDI version is more convenient, since you don't need to run a USB cable from the back of the keyboard to a plug in a wall socket, but that's just my opinion. I have no experience with either one.