The title of the book is the name of one of the main characters (which makes it easier to remember that character's name) and also tells us a lot about what we're supposed to take away from the novel.
The title tells us that this story is about Pedro, and also that, in a way, he is the story. Juan Preciado went back to Comala looking for his father, but all he got were these lousy stories. And the stories he received are what we, the readers get, when we pick up the novel.
It might shock you when you realize that the narrator who starts off the novel isn't the Pedro from the cover of the book. That strategy of catching the reader off-guard then forces us to ask the question "Okay, so why isn't the book called Juan Preciado? What's the big deal with Pedro?"
The answer is that this novel is all about Pedro. Juan is just the guy who collects and compiles all the old stories and gossip that lets us know the story of Pedro and the Páramo family. And Pedro is important because his decisions are the ones that have the power to make or break the town. And boy oh boy does Pedro break it.