The title of Paperboy is perfectly straightforward, just like our main man, Victor. It's the story of Victor's month as a paperboy one summer, and the title reflects that.
But why the word "paperboy" in particular? There are other words that might be used to describe this experience for Victor, after all—he grows up in all kinds of ways. It's becoming a paperboy, though, that sets everything in motion. Through the paper route, Victor meets all sorts of characters that he wouldn't come into contact with otherwise, and learns some hard lessons about adult life through observing other people and interacting with them.
He sees firsthand Mrs. Worthington's self-destructiveness and alcoholism, Mr. Spiro's love of knowledge and reading, and Mam's experiences with racism and prejudice. All of these things push Victor outside his comfort zone and help him to grow up… which makes the paperboy gig a pretty important marker in Victor's life.