I think you made a mistake in your question. The subject line says you're trying to transfer your 2021 tax return to use in your 2022 tax return, but the text of the question says 2021 for both.
Assuming the subject line is the correct one, I don't think you can transfer the PDF. You have to use the actual Turbotax software file. Turbotax does NOT read the PDF; that's just the actual forms that it fills out that you save for your records. There's a whole separate saved file that is a Turbotax file.
As far as the file being too big, I don't know what the hell that's about.
