Transfer ebook out of ADE from iPhone to MacAir

[Nathaniel Jonathan5E73]

I have an iPhone 6 (12.4.8.) and an older MacAir (OS X El Capitan 10.11.6.). The Macbook does not allow to install ADE; only the iPhone does.

Successfully downloaded a protected e-book with ADE on the iPhone.

How can I get it on my MacAir now? 

I'm even a paying Adobe customer with username & PW on some other computers at work. But as said, my own MacAir is not supporting it.

Thank you for any ideas!