The abbreviation DDS stands for doctor of dental surgery, and PA refers to a professional association. DDS is the degree held by the dentist, and professional association is a legal term that allows a person to be treated as a business entity for tax purposes.
PA is also an abbreviation for "periapical," or single dental x-rays taken two or three at a time to identify problems with teeth. These are different from bitewings, which show the entire mouth.
PA may sometimes be replaced with PC, which stands for professional corporation. This legal term refers to a business that pays taxes at the corporate rate as opposed to an individual rate.