Always On Availability Groups as a Secondary Read Only Database
The secondaries can be used as read only. Keep in mind data is still being added to the secondaries in near real time. 
Transaction log backups are conducted against the primary AG instance. 
If the secondary can not keep up with the volume of transactions coming across, it can cause the transaction logs on the primary to fill and continue to grow. 
In the event the primary becomes unavailable, traffic would then transfer to the secondary. This is the point of Availability Groups. 


If you just want a copy of the database to run reports against, you might look into setting up log shipping to another instance. It's a cheaper, and simpler solution than AGs.