Seconded. I have bad vision; I would love to use this to resize the fonts in single-column PDFs that I read.

Equivalently, I would love to reduce the column width to make zooming easier.

I prefer two-column format specifically because the columns are typically much narrower (less words per line), so I can zoom them to fill the screen and read much more comfortably than a single column document with 50 words per line.