I have long hated reading PDFs on my computer. Especially 2 column papers. PDFs often require scrolling up and down, back and forth, in order to read.
On a traditional PC or laptop, this requires use of scroolbars and mouse, sometims keys.
The PDF reader, e.g. Acrobat, sometimes allows you to just use arrow down in a text flow, but it often jumps around disconcertingly.
Reading it on my touchscreen tablet makes PDGs almost tolerable. Direct manipulation to move what I want to look at around is much better tnan scrollimng.
I still wish it was a single directional flow. But I can live wth this.
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I conjecture that if the PDF reader just allowed mosuing to drag around the viewport, like Google maps, it would be better, almost as good as touch.