Saving Annotation (ink) on permanently on PDF

page.FlatAnnots(); will not work with a professional license (premium license is a must).

About Mac, it ignores the annotation lock feature that's why it's not working.

So the only solution is to you page.FlatAnnots

Can you please share some premium license credentials ? I will be able to test the desired functionality and upgrade my license.

You can use the demo premium license, you need to give your project com.radaee.reader as a package name.
For the license info, please check Test different license types on Android before buying the license