I want to keep Google Drive's PDF reader open in the background. I found this question which solves my problem.
However, I do not see the lock–unlock button left of the X in the corner of my apps. Additionally, I have a split screen feature blocking some of the screen. I have attached a screenshot.
I am using a Moto X Pure with the recent 7.0 OTA update. Maybe my phone's new stock ROM doesn't have this feature?
Furthermore, the asker of the question I linked in the beginning was using Marshmallow. When my Moto X Pure was running Marshmallow, I did not see this feature either.