Pennsylvania defines "doing business" in the negative. That is, the Pennsylvania Associations Code (the "Code"), at 15 Pa.C.S. §4122, defines what minimal activities by foreign business corporations do not rise to the level of "doing business" for purposes of requiring registration. This section of the Code was repealed by the Association Transactions Act, effective July 1, 2015, replacing the exceptions with generally similar language appearing in new 15 Pa.C.S. §403.
Activities in excess of the types described in that section of the Code may constitute "doing business" in Pennsylvania, and registration would be required. Some examples of what does constitute doing business are: approving within the State sales orders of products or services, or providing repairs or maintenance within the State to products sold.
The determination of whether or not an entity is required to register under the above provisions is not a determination of whether or not an entity must file returns or pay taxes in Pennsylvania. Additional guidance as to what constitutes doing business can be found in the PA Department of Revenue Corporate Tax Bulletin 2004-01.