What does the pearl represent for each main character?
In my opinion, the pearl represents freedom for each of the main characters. The pearl could also be said to represent rebirth, which would be appropriate symbolically; pearls are formed when grains of sand are reborn as something rare and valuable. Each of the main characters would use (ideally) the pearl to redirect his or her life. Kino and Juana would use the pearl to create their own rebirths as people with a chance at a successful, comfortable future for themselves and their son. The doctor would use the pearl to become reborn as the man he prefers to be. The idea of rebirth also goes along with the idea of freedom, since rebirth would allow each of the characters at least a degree of freedom from some obstacle (real or perceived) in their lives.
When the family first finds the pearl of the world, Kino and Juana both view the pearl as a way to help cure Coyotito. This quickly begins to develop at Kino understands that the wealth he could acquire from the pearl might change his life. He wishes for marriage and Coyotito to go to school but in reality the pearl encompasses Kino and his thought. Kino kills a man because of the pearl which turns the pearl from a symbol of hope to a symbol of greed.
In the case of Juana, she begins to see that the pearl is changing her husband. Their once perfect relationship is crumbling and she wishes only to throw the pearl back into the ocean. To Juana the pearl represents the downfall of her family - specifically her husband and finally the death of her son.