How to turn on Horizontal scrolling

[Google Code Exporter]

You currently, like all pdf readers, have vertical scrolling set as standard.  
My preferred style of reading with my apptab 10 inch tablet, is horizontal 
scrolling whilst in portrait view.  Your settings menu appears to offer only 
vertical scrolling whilst also containing this phrase: "to scroll horizontally 
you must first move  horizontally 1/5 of the screen width".

I am afraid I do not understand what this is instructing me to do.  How do I, 
step-by-step, in laymen's terms, turn on horizontal scrolling?  Will it work in 
portrait mode?

May I suggest you simply offer the mode in the settings instead of requiring 
some special action that might be hard to describe?

Thank you.

Original issue reported on code.google.com by [e-mail] on 4 Jun 2014 at 5:57