With an estimated 96 per cent of the votes counted, Mr Biden had opened up over a 14,000 vote lead over the Republican incumbent.
Should Mr Biden win the Keystone State, he will have 273 electoral college votes – taking him past the magic 270 number needed to secure victory.
Earlier on Friday the state reported it had 50,000 mail-in votes still to tabulate.
Exit polls have shown Mr Biden has a healthy advantage among voters who decided to post their ballots early – which explains why he has been consistently chipping away at the president’s lead in Pennsylvania.
Workers in Pennsylvania have been methodically counting around 2.6million postal ballots received this year – a vast number prompted by concern among voters about standing in long queues on election day while the coronavirus pandemic continues to rage, having infected more than 100,000 Americans in a single day this week.
Mr Biden actively encouraged supporters to cast their votes early. Mr Trump, on the other hand, has repeatedly claimed that mailed-in ballots are open to widespread fraud, leading a higher proportion of his supporters to turn up on election day.
In the days since polls closed on Tuesday, Mr Trump and his surrogates have tried to halt the counting of votes in key swing states such as Pennsylvania and Georgia, citing baseless allegations of electoral fraud and filing a string of unsuccessful lawsuits.
“If you count the legal votes, I easily win,” he said, without evidence, before unspooling a series of conspiracy theories about mail-in voting.
But on the ground, officials involved in the process remained level-headed.
“There’s still some to count,” said Kathy Boockvar, Pennsylvania’s secretary of state. “So they are working incredibly hard. They are going to keep counting into the evening, and stay tuned.”
“We are going to be counting every ballot.”