It would very useful if we had a preview mode on the e-ink screen that would open the file on the LCD side allowing us to adjust zoom level and position and then once we are done we can send this view (zoom and position) of the pdf to the e-ink. This would be very convenient for complicated pdf with multiple columns etc where sometimes it is difficult to adjust because of the slow refresh rate of the e-ink.

In fact, interaction in the other direction would also be useful. For example having a button on the e-ink that would open the pdf on a pdf viewer on the LCD screen would be very useful. Specially if it can open the file at the right page number and zoom level. I would be an alternative to the camera button.