I can view PDF docs side by side simply by opening up the first document, Doc 1, clicking on Window, selecting Tile vertically and then opening up the second document, Doc 2, clicking on Window and selecting Tile vertically. Then both PDF docs are displayed side by side. Under Preferences/General, I do not have the box checked for Open Documents as New Tabs. I don't know if that makes a difference or not, I just never had that one checked. If there is an easier way, I don't know it. This is the way I've always done it. My version is Acrobat Pro 2017. If you prefer to look at your docs horizontally, then you can click on that. I prefer the vertical view.