You should be transferring the 2020 tax data file to the 2021 TurboTax program not the PDF. The tax data file has a file extension of .tax2020 and is stored by default in the TurboTax folder within the Documents directory.
See this TurboTax support FAQ for transferring last year's return to 2021 TurboTax for Windows - https://ttlc.intuit.com/turbotax-support/en-us/help-article/import-export-data-files/transfer-last-y...
If for some reason you Only have the PDF and Not the tax data file, see this TurboTax support FAQ - https://ttlc.intuit.com/turbotax-support/en-us/help-article/import-export-data-files/transfer-last-y...
