You can transfer saves across two ways to/from Mac, either via iCloud or via Finder.

Transfer via iCloud:

Turn on iCloud syncing for Divinity 2 on iPad (by going to Settings → Apple ID → iCloud, then at the bottom of the list toggle syncing for Divinity 2 on).
Make sure that Divinity 2 has its “Store profiles and saves in iCloud” option toggled on (Settings → Divinity 2 at the bottom of the list).
Check if the save files in Divinity: Original Sin 2 Saves folder synced correctly by opening the Files on iPad and selecting iCloud Drive.
Synced saves will be available on other Apple devices with iCloud syncing toggled on for Divinity 2.

Transfer via Finder:

Connect your iPad to iMac or MacBook directly using a USB cable.
Open Finder and select the iPad in “Locations” (“Trust” the device if the prompt asks you to).
Open the “Files” tab and expand the “Divinity 2” app.
Drag and drop the “Larian Studios” folder over to the Desktop, this folder contains your save files.
Add saves to the folder (...\Documents\Larian Studios\Divinity Original Sin 2: Definitive Edition\Player Profiles\<ProfileName>\Savegames).
Drag and drop the “Larian Studios” folder back to the “Files” tab inside the Finder window.

You can transfer saves across to/from PC via iTunes:

Connect your iPad to your PC directly using a USB cable.
Open iTunes then open iPad view.
Open “File Sharing” in the “Settings” list and select the “Divinity 2” app.
Click save, and save the “Larian Studios” folder over to your Desktop, this folder contains your save files.
Add saves to the folder (...\Documents\Larian Studios\Divinity Original Sin 2: Definitive Edition\Player Profiles\<ProfileName>\Savegames).
Drag and drop the “Larian Studios” folder back to the “Divinity 2 Documents” window.

Collector's Edition

Can I cross-play with PC/Mac or other consoles on iPad?