72 Pa. Stat. § 720
Every incorporated company or limited partnership subject to the payment of tax on its net earnings, under section twenty-seven of the act, approved June first, one thousand eight hundred and eighty-nine (Pamphlet Laws, four hundred twenty), entitled "A further supplement to an act, entitled 'An act to provide revenue by taxation', approved the seventh day of June, Anno Domini one thousand eight hundred and seventy-nine", shall annually, upon the fifteenth day of April of each year, report to the Department of Revenue, setting forth the entire amount of net earnings or income received by such corporation or limited partnership, from all sources, during the year ending December thirty-first preceding. Every such incorporated company or limited partnership, at the time of making every report required by this section, shall compute and pay to the department the tax due to the Commonwealth upon its net earnings and income, as required by law.
72 P.S. § 720