In the native reading app on Onyx Boox readers you can choose to show one column at a time. The text in that column would then be zoomed in full screen, making the text bigger. If all or most your textbooks are in that format, I think it would be easy to read on a Note Air device.
Here's one video where the article mode is shown with the PDF divided into 4 regions:
https://www.youtube.com/watch?v=w5HE01amvdc.
EDIT: I found the article used in the video. Dividing the paper, consisting of two columns, into 4 sections in portrait mode made the text comfortably readable even on my Nova 3 (7.8" screen).