The DDFL file extension is related to the DiskDoubler a compression tool used to save space on Mac drives, developed by Symantec.
DDFL file extension - Apple DiskDoubler file
What is ddfl file? How to open ddfl files?
File type specification:
The DDFL file extension was used by DiskDoubler (DD, a data compression utility for compressing files on the Apple Macintosh platform. Unlike most such programs, which compress numerous files into a single archive for transmission, DiskDoubler was intended to compress single files "in place" to save space on the drive.
When such a file was opened, DiskDoubler would decompress the file before handing it off to the application for use. A later addition, AutoDoubler, added background compression, finding and compressing files automatically when the computer was idle.
It is obsolete tool without any support from developers (Symantec).