Your manual is saying, the two shafts; the Tractor PTO shaft and the snowblower input shaft should be parallel. How you go about doing this is left up to the reader. If you can imagine the snowblower sitting at rest on the ground and you tightened or loosened the top 3pt link, it would tip the snowblower back and forth changing the angle of the input shaft. So I adjusted it as best I could by measuring the distance from the frame to the ground in different locations, hoping the frame is square in relationship to the input shaft.
I don't think my blower has adjustable skid plates, this must be an option. But I think we are saying the same thing, just going about it differently. I just took a good look at my blower and I don't think the input shaft is square with the blower frame....so my method isnt going to work. If I had a short level, I would but it on the PTO and then the input shaft and adjust the top link until the bubble is in the same place on both shafts. I don't how else you can get the two shafts parallel.