Pressing Ctrl +/- (on numeric keypad) on a fullsize keyboard is able to zoom in / out. But on a notebook keyboard without numeric keypad, no possible to do this.
Please consider to support pressing Ctrl +/- (on non-numeric keypad, i.e. next to backspace key) to do the same.
I use a mini keyboard with no number pad and can do this on Windows using Ctrl and the two keys (+/-) to the left of Backspace. Ditto on my Win laptop.
Ctrl +/- works with me on pl3 and win 10. If the image window has focus than just scrolling is working too.
More annoying is that when the screen or program window is to small that the zoom info is disappearing while the Viewpoint items stay vissible. Zoom info is something I need all the time, viewpoint I only use once in a session.