Oh I didn't know that! In fact while HRTC buses buses find their names everywhere from bus stations to websites, I personally felt that HPTDC doesn't have THAT elaborate a publicity, for a first time visitor like me couldn't find any means to book it offline!
Nevertheless, thanks for adding another option to my knowledge. (y)
PS: At the same time in a different context I'd say HPTDC's residential hotel 'The UHL' at Joginder Nagar impressed me to the core. Pretty decent services at reasonable prices.