How to prevent Google Play Books from uploading ebooks? Even with Enable PDF uploading disabled on Google Play Books application every book I add gets added to the cloud.

Is there any way to disable this behaviour?
Google assumes that you would be using at least two devices to read a book at some point of time, so it only provides a feature to upload the ebook and then read it, or optionally download it to read without internet. There is no option to directly read the ebook without uploading it. 

When you tap an Ebook in your file manager or in Downloads app, and provided that you have other ebook reader apps installed with none selected as default, Google Play Books offers the option to open it with heading "Upload to Google Play Books" which overrides the setting anyway. 

So the solution is to compromise with the functionality or switch to an alternative.