Getting my default icons back on in the middle top icon area

I don't know what it's called, but I clicked an icon in the middle area at the top of my document where I can say what page I want to go on, and now only four icons appear to the right of where the page selection is. I used to be able to click and icon and say, "Fit to width scrolling," but now that button has disappeared, and I have tried everything I can think of, even uninstalling and reinstalling, but nothing works. Now the area looks like this. How do I get my default icons back? I'm on Windows 10 if that helps.