My impression, being in PA all my life, is that the inclination of people was to not do mail-in or early voting, because it was new this year. It was also pretty darn complicated to figure out.
But Democrats made a huge push to get their people to do mail-in voting here. The 80% slant in early voting to Biden is maybe a little high, but very much in the ballpark of how much more I’d expect it to lean to Biden. At least 70%.
I doubt there is another state in the union outside of PA where Biden will get a bigger share of the mail-in/early vote. (We still don’t have real early vote, it was just dropping off your mail-in in person at pretty sparse locations.) Republicans had absolutely no inclination to do the mail-in voting here, but the Democrats made it a huge campaign push for their side on the ground.
The mail -in ballots are new, not like absentee. I used one, dropped it in a drop box. I know other Republicans who did the same. Much easier than standing in line, or having something go wrong on Election day.