How do you remove the background of a presentation?

I am a student and all my PowerPoint lectures are sent to me before they begin. I take those lectures put them into keynote then use "Open in another app" to open as a PDF in notability.

Notability allows me to record the audio of the lecture and take notes directly on the slides. However, some teachers use dark or colourful backgrounds that make annotating them a bit frustrating. I want to remove the backgrounds in the same way I could if I selected print "With Backgrounds: off"

Just to be clear I do not want a hard copy of the presentation. I want to have a digital version without the background that looks similar to the hard copy version keynote makes when you select print "With Backgrounds: off"

Thanks