From the PDF you cite:

"Since the AUC is a portion of the area of the unit square, its value will always be between 0 and 1.0. However, because random guessing produces the diagonal line between (0, 0) and (1, 1), which has an area of 0.5, no realistic classifier should have an AUC less than 0.5."

So it appears that not 0.8 but 0.5 is the randomness threshold; therefore 0.7 is not so bad.