Below are the 6 official tests released by ACT over the years in electronic format. ACT does not release a new online test every year, which is the reason for the gaps between the years.
When attempting these tests, start with the most recent one and work your way back in time. The answer key is at the back of each document.
Math: Note that the Math section has gotten progressively more difficult over the years, so the 2018-2019 test is a better representation of today's Math section than any of its previous counterparts.
Reading: Another recent development is that the Reading section now has a paired passage; this appears only in the two most recent tests (2018-2019 and 2015-2016).