.bac is Backup File
Key Features: .BAC File Extension
What is an .BAC file?
Backup of a file or group of files created by Backup Plus, a file backup program from Avantrix; may be a standalone backup file or part of a split archive, which uses the ".b01," ".b02," etc. extensions; can be restored using the Backup Plus Restore Manager.
The Backup Plus software is no longer supported. If you have a single BAC file that is not a split archive, try renaming the file extension to ".zip" and opening it with a .ZIP decompression utility, such as WinZip. If you have a split archive, try also renaming the split archive files (".b01" to ".z01", etc.) and opening it with WinZip.