PVC are measured in nominal pipe size, where the OD and ID are only roughly the size listed (1 inch, 2.5 inch, etc). Acrylic pipes sized based on true ID or OD, depending on the seller (some list ID, some list OD).
ID = inner diameter
OD = outer diameter
I just finished a big project with acrylic pipes where i tapered down 2" acrylic pipes to fit into PVC schedule 40 fittings. its not that hard, but requires patience and an even source of very hot air (heat gun).