According to The Law Dictionary, the p.p. signature indicates that someone is signing a letter on behalf of another person. The p.p. derives from Latin and stands for per procurationem.
According to the Law Dictionary, p.p. is used in commercial situations where an office employee must sign a document or letter on behalf of a management. Presidents and managers are frequently too busy or unavailable to sign documents personally. Consequently, individuals must appoint a representative who is permitted to sign documents on their behalf.