Even before the election was held, we knew that there were going to be a lot more Democrat mail-in ballots than Republican. Like, here are the Pennsylvania numbers, from a webpage archived the day before the election:

https://web.archive.org/web/20201103061959/https://electproject.github.io/Early-Vote-2020G/PA.html

So Biden was already up by 1.1 million returned absentee ballots by election day.

Here are the final numbers.

https://electproject.github.io/Early-Vote-2020G/PA.html

I guess it went up because ballots were returned on election day (or because there's some lag in the reporting). Biden got 106k more and Trump got 68k more:

In theory, I guess someone could have printed those final ballots. But Biden only gained a net of 38k votes. And his overall lead is 81k.

Even if you only used the mail-in ballots from the day before the election, Biden would still win by 43k.

Not an exact analysis, because you need to know how independents voted. But, makes it clear to me that it wasn't just somebody printing fake Biden ballots until he won.

Does that seem convincing to you?