Definitely the two columns for me. It's super annoying skimming a paper and having to scroll down and back up again in a zig-zag pattern.
While it looks perfectly fine on a phone. Two columns layout looks terrible on a smartphone, the text is too tiny to read comfortably.
It would probably be even better if you can flip it left and right like a ebook instead of scrolling to allocate the content faster. But current design is good enough IMO. (Compare to reading a pdf on cellphone)