DD, DiskDoubler Archive (.dd)
The DD Archive file format
DD files are created when using Macintosh's DiskDoubler. DiskDoubler is a file archiver for older Apple Mac operating systems. The DD files contain files that have been compressed for archive creation and space saving reasons. DiskDoubler and thus the DD file extension are now deprecated. The program has been replaced by Now Compress and Stuffit and can not be run on Mac OS X.
Technical details of DD files
The Macintosh DiskDoubler uses a compression algorithm called LZ78 to compress and archive files from a Mac computer. The program compresses files on a user's hard drive. When opened, the compressed DD files expand immediately.