What would be the best way to connect an Iphone or Ipad with the iOS app called PdParty to a Pd patch on my Bela Pepper?

You could connect your Pepper to a local network, either via a wifi dongle or an ethernet cable. Then you could send control messages of some sort between the two, e.g.: OSC or raw UDP messages.
Or you could try to connect Pepper to the iPhone via a USB cable with a lightning-to-USB-B cable (and/or adapters) then it would show up as a MIDI device and you could send MIDI directly from one to the other.