Hi,
i have the same problem, only way I found to restore the previous zoom level is to open any note whit a pdf attached and zoom back by CTRL + Mouse wheel.
No way to restore it from a note w/out pdf: you can zoom in/out by CTRL +/- but, as Catie911 said, just to a limited size (2 steps zoom out and 5 steps zoom in from the level you are, in my experience)
I wonder if there's a default zoom level and a way to immediately restore it.