How to read 2 or more documents side-by-side in Android? When I do my math exercises, I need to view the book and the exercises at the same time. Sometime I need to open a third PDF, but it's more optional. How can I do so with Android? On iPad I use the application Side by Side, which allows to read up to 4 different PDFs side by side.

I use Nexus 7 with Android 4.4.2.
Try using floating apps. Multi window feature is only available on a few android devices like samsung Note, etc.

Floating apps will provide similar functionality. Try this:

https://play.google.com/store/apps/details?id=com.lwi.android.flappsfull