You may also look here: How do I run two programs at once in split-screen?
Tested on a Samsung Note 10.1 (SM-P605) with Android 4.3 i found some apps which are supporting multi window, e.g:
- My Files (*)
- stock Video Player
- MX Player
- stock email app
- Gmail
- stock Webbrowser (*)
- Chrome
- Gallery
- Contacts
- Phone
- Google maps
- Polaris Office 5
- WhatsApp
- Facebook
- Watch now
- Chat on (*)
- Google Play Store
But there are really very few apps capable to run with multiple instances (from the above list only those which are marked with (*)).
So, you can for example open My Files
twice (left & right of upper & lower half screen):

Unfortunally, Polaris Office 5
(pdf viewer) isn't capable to run twice. So if you want to view 2 pdf docs side by side, it seems that you have to install a 2nd pdf viewer app and e.g. open Polaris Office 5
on the left and the other pdf viewer on the right side of the screen. A short look to Google Play Store for pdf viewers that support multi window:
At the following screenshot I had opened Ivan Ivanenko's PDF Reader & Ebooka PDF Viewer side by side:
