Since a pdf only transfers limited info like name and address you are not saving much by transferring. If you can't transfer it just start a new 2020 return.
Where did you get the pdf? Are you trying to transfer it into the online version or the Desktop program?
You need to have the actual electronic pdf file not a printed or scanned copy. And then it only transfers your basic personal info. See the last section here. Or you will just have to manually enter any it needs as you go along.
Transfer pdf to Online version. See the section if you used other software last year.
https://ttlc.intuit.com/community/entering-importing/help/how-do-i-transfer-last-year-s-return-into-...
