On the Downpour app, you can download the audiobook for offline listening. If you want to download the DRM free files you can also login to their website using desktop or desktop browsing mode and download it from there.

It is worth noting that I have heard, "While much of Downpour's content is DRM free and allows for usage across platforms, select products on Downpour are required by publishers to have DRM-protected files." 

I have not come across any yet that were not DRM free.