I would take POST data, and plug it into a templating system (like Django's), which is using something like LaTeX to define the basic properties of the document and populating with data and logic from Django.

Basically a mix of two markups in one file. Meh, there's probably a better way to do that.

Eh, easy. http://jspdf.com/

You can create a file with javascript at the client side, without creating load on the server.