From Guns.com:

“My Office is taking the initial step of clarifying – through my official, legal opinion – that under Pennsylvania law, 80 percent receivers are firearms and can be treated, regulated, and enforced as such,” Shapiro said at a news conference with Wolf and State Police officials.
The six-page opinion, issued to Col. Robert Evanchick, the State Police Commissioner, concludes that a partially-manufactured receiver is a firearm if it is designed in a way that it “may be readily converted” to a completed receiver. This would result in such items being treated under the law as factory-produced firearms, with serial numbers, federal background checks and the like.