How does this circuit give DC output?
Edit: 
As drawn, this is a clever synchronous rectifier using BJTs as very-low-drop diodes. It would be suitable for a measuring instrument since the voltage drop is very low (no diode drop loss). A full-wave rectified output appears at the terminals. 

If you reverse it (the way you'd normally draw it with flow from left to right), you get this: 

This circuit is called a blocking oscillator. The input is the DC and it produces a kind of square wave high voltage AC at the output of the transformer. 

It depends on either the transistor coming out of saturation (due to insufficient base current for the current at the collector, which increases with on-time) or the transformer saturating (due to core saturation due to the high collector current) for the turn-off. 

They're not very well controlled or very efficient circuits, but back in the dark ages, I made some simple inverters that were not much more than a 'filament transformer' a couple 2N3055s and some resistors (plus a snubber) (similar to this) that could run small AC-powered measuring instruments from a 12VDC source.