MainStage 3.5 Score display

In MainStage, click on the Help menu and then click on the MainStage Help option in the dropdown menu. When the MainStage User Guide is displayed onscreen, do a search for Lyric Sheet. The answer to your question is in the first search result "Use screen controls to display PDF document pages in MainStage"

You’d just have to change your layout so your scores maximise the screen real estate rather than controls. I think a better option is to use the ForScore app (or equivalent app with decent MIDI capabilities) on an iPad and have it control MS...but that’s just how I do things. If you have a decent-sized screen on your Mac and don’t need a bunch of controls, having your scores in MS is still a viable option. At least it’s giving you an option.

Why not? If your score is a PDF then this is a great option.