If you do use the Online software to prepare...even if you print and mail it after the deadline....the Online software will shut down entirely sometime after the 25th of Oct (sounds like maybe the ~30th).
When that happens, your Online account will be locked entirely for a couple weeks or perhaps a month+, while they transition to the 2019 software.....during that lock-down period, neither the 2018 PDF nor the .tax2018 data file will be available for you to download, even if you do have the desktop software to continue working on it. You'll need the .tax2018 datafile to continue working on 2018 with desktop software......plan accordingly to get that datafile well before the lock-down if you have been using "Online" software for your 2018 data....or download a copy of the datafile after every session you enter some new data, starting say..the 24th of October.
Either that, or start working with the 2018 desktop/download software now....then it won't be an issue.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
