The one-column format is fine on a large monitor, but on a small phone I prefer narrower columns, because a wide column would either make the text too small or it would require horizontal panning while reading.
So I consider the two-column format as better for phones, not worse.
I frequently read documents with many thousands of pages, which also contain many figures and tables.
A variable layout, at least for me, makes the browsing and the search through such documents much more difficult.
I have never ever seen any advantage in having the text reflow to match whatever window happens to be temporarily used to display the text, except for ephemeral messages that I will never read again.
For anything that I will read multiple times, I want the text to retain the same layout, regardless of what device or window happens to display it. If necessary, I see no problem in adjusting the window to fit the text, instead of allowing changes in the text, which would interfere with my ability of remembering it from the previous readings.
I really hate those who fail to provide their technical documentation as PDF documents, being content to just have some Web pages with it.