This page doesn't provide all steps for importing pdf - https://ttlc.intuit.com/turbotax-support/en-us/help-article/import-export-data-files/transfer-last-year-return-turbotax-online/L279eUvY2_US_en_US
A pdf will do so little it is not even worth the trouble. A pdf will only put in some of your personal information like name, address and SSN. You can easily key that stuff in for yourself. A pdf will not transfer employer information, etc. etc.
In addition to importing personal information, I expect TurboTax Online to show side-by-side comparison of last year vs this year if it has previous year's data. I am using TT for the first time and loved that feature of TaxAct.
Transferring from a pdf file is limited. And you can't transfer from a 1040SR pdf. You should try to Transfer from the .tax file. Here's a couple links to explain more..... https://ttlc.intuit.com/turbotax-support/en-us/help-article/import-export-data-files/transfer-last-year-return-2021-turbotax-windows/L0JcyjyrG_US_en_US?uid=lqyfj3a8
PDF vs .tax file