Yes, you can use iCloud to sync the digital planners across devices.
Here are some dedicated tutorials:
a) if you are using Goodnotes 5 – check this tutorial
b) if you are using Noteshelf – check this tutorial
Some of my clients also connected Noteshelf with Evernote to see the files an all their devices (inside Evernote)
c) if you are using Notability – check this tutorial