Okay, I get it. They are saying that the DC component of the unfiltered rectifier output is .9 times the Vac rating. That makes sense, but they should not call it an average DC voltage. It's misleading.
In actual use you will get somewhere between the .9 and the 1.41 depending on the load and the capacitor size. As they say, it's better to arrange to have as little ripple as possible (big cap or underrated load) to increase capacitor life.
I would seek another source of information on all this. There has to be a good book out there, or even web page on power supply design. This little paper does not appear to be the best way to learn about it. Also, the end use really defines how the supply should be designed and rated.
Do you have an end use in mind or is this just to learn about it?