what is the operating screen, the app you reading on and the model of the phone ?
the reason this book being ~20 mb with 750 pages is that %99 percent of the things inside is vectoral, including the variable font that I used. While trying to find the perfect page aspect ratio and the perfect variable font width, I tried it with many different screen sizes, operating systems, dpis etc and clearly tell you that you have another problem.
But instead of troubleshooting my case, just tell what the ideal client program that breaks the curse of PDF is! I'll even try the Acrobat monstrosity if it's the only one