The interface is quite bad. I was able to scroll right to see a total of three slides, with no indication that there were any others. I thought that was the end!

I tried hitting space, and another slide scrolled in! But I wasn't done reading, so I hit shift-space, which is the common idiom to reverse the direction of space. But it scrolled in the same direction. Now I'm two slides behind. After some more thrashing, I found that I could use the arrow keys to navigate. Delete also goes back.

Please don't make me use trial and error to figure out your UIs, Google!