here is a picture/screen shot showing scribbling in the margin of a pdf magazine using the Boox 60.
i did exactly what you say- zoomed in, scribbled and then zoomed back out. Sorry about my poor chicken scratching
. I do this often for schedules.
Like when I go to CES I have a list of things I want to see and the booth numbers in a pdf downloaded from the CES planner. as I go along I scratch off each one or write a note about it. I also add things that I want to go back and look at again etc.
This was taken from the thread here
https://www.mobileread.com/forums/showthread.php?t=91525