Transition PVC outdoor conduit to EMT
You want a PVC-to-threads adapter as depicted below (presumably 3/4" to match the size of the knockouts on your box, with a 1" to 3/4" PVC reducer used to adapt the LB's 1" to the adapter's 3/4").



You then use PVC from the LB to the adapter/reducer pair, and a locknut to hold the adapter to the box.