You need to transfer (not open or import) from the .tax2020 file (not a PDF) and the .tax2020 file must be on your hard drive not a removable storage medium.
When using the 2021 TurboTax software you will be starting a New Tax Return and Transferring the 2020 tax data to the 2021 tax return. So you only need to browse to the location where you copied the 2020 tax data file to transfer to the 2021 tax return.
