Comment by severine

I think it's had it for quite some time, it's at:

BOTTOM MENU > Aa > Reflow

It's in the manual:

PDF documents - Crop margins automatically or manually - Reflow documents to extract text and read easier on a small screen - Different zoom types and page flow directions to read multi-column documents easily - Auto straighten tilted documents - Apply OCR - Special Panel zoom feature for reading mangas

https://koreader.rocks/koreader-user-guide.pdf