They do not appear to have been harmed by Harry's Patronus:
"You gave Mr. Malfoy quite a fright," said Lupin.
Harry stared. Lying in a crumpled heap on the ground were Malfoy, Crabbe, Goyle, and Marcus Flint, the Slytherin team Captain, all struggling to remove themselves from long, black, hooded robes.
Harry Potter and the Prisoner of Azkaban Chapter 13: "Gryffindor Versus Ravenclaw"
They do appear to have fallen over, but whether that was due to a direct impact with the Patronus, or whether they simply recoiled in fright and lost their balance is unclear. For whatever it's worth, Pottermore does suggest that corporeal Patronuses1 have some bodily substance (emphasis mine):
The incorporeal Patronus is not a true Patronus and while it will give limited protection, it cannot provide the defensive power of the corporeal Patronus, which has the form and substance of an animal.
Pottermore Patronus Charm
But it is of course not clear whether Harry managed a corporeal Patronus in this instance.
1 Patronii? Pantroneese?