Imitate letters in text falling down as if under gravity AE
If the text is random characters, you could possibly do this using a particle engine.  Assign one character per frame to a comp, then apply those frames to the particles texture... Something like Particular from Trapcode would work with this.

Alternatively if it's a full page of actual text:
I reckon Newton 2 could do this..
http://www.motionboutique.com/newton/
Or Cinema 4D, but I don't think the free version that comes with AE now has the physics plugins included.