pdf - 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.