As far as reading a PDF page from a manual on the Amazon Fire, conceder this;
Wiki -
Kindle Fire
"Excluding its external border the screen is a little larger than a 4x6 picture."
Reading an e-book would be just fine with those dimensions. Although some may want, or need, a larger screen.
Think about it this way, imagine an orchestra trying to read a PDF of sheet music from a 4 x 6 screen.
I'm not disappointed in the Amazon Fire, I may even get one for reading. It's just not what I need.