From the Voting in PA website. This is the law under act 77.

Deadlines for the November 3 General Election

• October 27 at 5 pm - APPLICATIONS for a mail-in or absentee ballot must have been received by your county election board or other designated locations
• November 3 - Deadline for returning your voted mail-in or absentee ballot.
  • IMPORTANT: Voters who have not yet cast their mail or absentee ballot are urged to hand-deliver their voted ballots as soon as possible to their county election office or other county drop off location or drop box.Ballots must be delivered by 8pm on Election Day but the sooner the better. Voters should no longer use postal service delivery as it is too late to guarantee your ballot will be delivered in time.
• Learn more about upcoming deadlines

Missed the deadline? If you have an emergency (such as an unexpected illness or disability and last-minute absence) you may still be able to get a ballot after the deadline. Find information about how to get an emergency absentee ballot at the end of this page.