My 2020 return was filed with TurboTax from a disk.
See this TurboTax support FAQ for transferring the 2020 tax data to the 2021 online editions - https://ttlc.intuit.com/community/entering-importing/help/how-do-i-transfer-last-year-s-return-into-turbotax-online/00/25953
When I upload a pdf of my 2020 return, nothing is read from it. And I cannot upload a file with the .tax2020 on it.