Why is the PNP transistor used in this circuit?
That's because NPN would not work. Try simulating the circuit in a simulator.
PNP is used in the high side drive because it can switch the output properly to VCC, just like NPN is used in the low side drive because it can switch the output properly to GND.
The Vbe is 0.7V, so if you try to set D1 high, Q6 will clamp Vb it to 0.7V, and because Q3 also has 0.7V Vbe, Phase1 is 0V.