Open DNF File
DNF extension files are commonly associated with Duke Nukem map files. Basically, DNF file is a Duke Nukem Forever Data File.
Files with the DNF extension are thus classified as game archive files.
Duke Nukem Forever is a first-person shooter video game for Microsoft Windows, OS X, PlayStation 3 and Xbox 360, it was created by 3D Realms, a video game publisher, developer and distributor established in Garland, Texas in 1987, whose main shareholder was Action Entertainment Software Inc.
3D Realms developed the DNF file type in Duke Nukem Forever as a map file.
For more information about these types of files, and the games involved, you can go to the link: