That would be Intuit Link.

From 2017, Link is integrated with PTO.  You access that from within PTO.  In the left panel, click on "Intuit Link".  The next screen will show a list of active invitations you have for the respective year.  You may click on the dropdown for year to select 2017, 2018, or 2013-2016. Click on "View Request" under the "ACTIONS" column and a drawer will be opened to show all the items related to that particular return.

If you would rather use the old portal to access the documents for years prior to 2017, the URL is https://link.intuit.com/accountant/login/#/signin.