By logging in to your Intuit or TurboTax account and navigating the "Your Tax Returns & Documents" section, it's possible to download your previous tax returns. Simply select the year, for example, 2017 or 2018, and select Download PDF to view and download the PDF you want. 

Unfortunately, although it allows you to manage and see tax documents for multiple years, it does not have a bulk download feature, so you'll have to do so one at a time. I usually open each year in its own tab and download in bulk that way. If you used the desktop version, you can also check your TurboTax folder (on your computer) for .tax2017 and .tax2018 files as well.