The Central File IS the .rvt file.
There are local copies that users have on their local workstations, when using worksets.
All users should STC ( synchronize ) and relinquish all worksets.
Then, the Central File should be sent to the engineer. The best way is via a WAN with a WAN accelerator on each end of the pipe.
cheers.............
Cliff B. Collins
Registered Architect The Lamar Johnson Collaborative Architects-St. Louis, MO