Do PCBs have schematics?
Yes, all PCBS have schematics. The ones that are public include


Open Source Hardware
Very simple designs are often based around an example schematic
provided in the data sheet of the primary IC. You can usually obtain the data sheet from the IC manufacturer. 


 Bus Pirate


  

AD603 Data sheet

However, as other answers point out, the majority of schematics for PCBs in commercial products will not be available to the public.