pdf files on split screen

You open the first PDF in Apple's Preview, and the second you right-click on and choose Open With Preview. That should have both PDF open in separate Preview windows.

With the first PDF document, click and hold the green traffic light in the title bar and choose Tile Window to Left of Screen. You now have half of your split screen effect done. Then just click anywhere in the second open PDF and it will automatically become the right-half of your split screen. To undo split-screen, press F3 and click on the left most icon in the galley at the top of the screen. That should represent your Desktop before the split-screen activity.

And If you happen to have the two pdf files in the same window, control-click on one of them in the window Sidebar and choose Open in New Window, then proceed as VikingOSX described.