According to The Law Dictionary, the p.p. signature occurs at the end of a letter and designates that someone is signing a letter on behalf of another person. The p.p. has Latin roots and stands for per procurationem.
The Law Dictionary notes that p.p. is used in business contexts when an office staff worker needs to sign a document or letter on behalf of a manager. Often times, those in higher positions, such as presidents or managers, are too busy or unavailable to sign documents themselves. Therefore, they must designate an authorized representative who is allowed to sign documents on their behalf.