P.C. stands for "professional corporation". It means that the lawyer has incorporated himself. A "professional corporation" is a particular kind of corporation that has certain statutory requirements - usually, only members of the licensed profession (e.g., medical doctors, lawyers, accountants) can have ownership interests (be shareholders). Spouses, employees, and others cannot be shareholders unless they are properly licensed to practice the profession. Like other corporations a professional corporation permits the shareholders to limit their liability for certain "misdeeds" - their personal assets are protected and only their corporate assets are at risk (this depends on the law of the particular state).
PC is for "professional corporation" which is the type of business formation that they have formed. It is a corporation, but specifically allowed for certain types of professionals. Some other types of business entities you will see are Partnerships, or PLLCs, professional limited liability companies. The business formation is merely a choice by the attorney for their business relationship. The specific type of business formation has no bearing on the credentials of the individual attorneys.
A Professional Service Corporation or PLLC is intended for doctors, lawyers, dentists, architects, and other professionals who must have a license to engage in their work. Some professions are required to form Professional Service Corporations if they are going to provide services through a corporation. We suggest that you review RCW 18.100.050, RCW 25.15.045 for PLLC's as well as the licensing rules governing your profession. Check link below to find if the business is registered as a corporation and to receive further information.