Yes you can transfer the .tax2023 file from desktop to online. And yes, you can save your 2024 as a pdf using online.
How to transfer last year’s return? https://ttlc.intuit.com/questions/1900883
If you used the Desktop program last year here’s How to transfer into the Online version. You need to have the prior year .tax2023 file on your computer. Expand the link if you used the Desktop software last year........
https://ttlc.intuit.com/turbotax-support/en-us/help-article/import-export-data-files/transfer-last-year-return-turbotax-online/L279eUvY2_US_en_US?uid=l8w7nlbv
If you used the Desktop program before you may notice several differences and may not like the online version. It has less features and no Forms Mode. The Online versions don't have all the same forms so you might have to upgrade to a higher Online version. You can only do 1 return. Your return is only saved online so be sure to save both the .tax2024 data file and the pdf file to your computer when you are done. In case you need to amend in the future or if you need a copy to get a loan or something. You might not be able to access your online return when you need a copy.