All of Pennsylvania uses some form of paper ballot:
As of the June 2, 2020, primary election, all 67 of the Pennsylvania's counties have deployed voting systems that produce voter-verifiable paper records and meet 21st-century standards of security, auditability and accessibility.
In April 2018, the Department had informed counties they must select voting systems that meet the new criteria no later than December 31, 2019, and implement the systems by the 2020 primary.
Having paper ballots allows validation by hand counting and hence makes fraud or deliberate miscounting extremely difficult.
This article details some of the problems that were experienced during voting in Pennsylvania. They were due to accidentally voting for too many candidates, having Sharpie markers bleed through, and the line. The number of problems were reportedly relatively low.
Another question here had to do with some erroneous reporting of early ballot tallies (don't recall if any was in Pennsylvania), but that had nothing to do with the final tallies.