Studies show that line lengths near or longer than 80 characters are more difficult for the eye to follow. The Spritz app sets a boundary around the words to be read so that adds to readability. The total number of pixels (rods and cones in your eye) compared to the number used to read the text field is likely much higher on your computer than on your phone. For example, if the pdf uses 5 to 15% of your visual field but your phone only uses 1 or 2% of you visual field, that is a big difference in processing load. The Spritz app uses even less of your visual field.

I deal with this processing load when I need to fill out a paper form by covering the text above and below with paper. Otherwise, the visual processing load can be overwhelming.

This forum works good because this Reply to Thread form has minimal text clutter. The worst clutter is the emoticons.

I used to be able to create reams of documents in MSWord. (Word 2003 or earlier) When the new MSWord with ribbons came out, the new visual clutter made it so I struggle to create documents. I am highly aware of visual processing load.