DR Site availability if primary goes down I have two SQL Servers at a production site (2 node WSFC) in an Availability Group, and one SQL Server at a DR site.

If I add DR site node to existing WSFC to configure AOAG:

If both servers at the primary site go down, will WSFC will be online or offline? Will my secondary replica still be available for read-only workloads?
I am using SQL Server 2014.

I am planning to create a file share witness for quorum configuration.

I just want to make sure if the whole production site is down, will my secondary replica at DR site still be useful for the users? Will it be read-only or read-write? As it is Async, will it require manual or forced failover?

All are SQL Server 2014 instances, and are stand-alones included in AOAG. Production site is synchronous and DR site is asynchronous.
Technically, yes, you should still be able to read from the DR instance assuming it is configured as a Read Only replica and you are connecting to it directly (As read only routing through the primary would not be working).

You should really be concerned in this case though about bringing the DR site online as the new primary replica.  You would need to force quorum on the DR site, and manually failover (with possible data loss) so you can then accept reads and writes at the DR site.

Perform a Forced Manual Failover of an Availability Group (SQL Server)