I filled the 2020 return online last year, but forgot to submit it. I am not sure if I paid for it, but I do have a pdf copy of the return. How can I get the return?

The only way to file a 2020 return now is by mail.   Print it, sign and date it in ink, and mail it.  Expect a LONG time for it to be processed.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.

Federal and state returns must be in separate envelopes and they are mailed to different addresses.  Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.

https://www.irs.gov/filing/where-to-file-paper-tax-returns-with-or-without-a-payment