It's definitely not the clutter issue (there is just as much text on my phone as on a pdf on my computer) but it could be the shorter line lengths! Interesting idea.. so you believe that it's more difficult for the brain to process longer lines of text? In that case I suppose I should attempt to shrink line lengths on my computer and see what happens.

Have you attempted to retrain your brain to read? I know you said you said you cope with stress but have you attempted to create any sort of exercise routine for yourself to improve the processing/reading?