A CDF file can originate from one of a few different programs. Typically, it could be a Computable Document Format file, an Affymetrix Chip Definition file, a CryENGINE Character Definition file or a Common Data Format file.
The easiest way to determine the type of a CDF file is to use a program such as File Viewer Lite to read the metadata of the file. The metadata indicates the program that generated the file and what type of data it contains. After determining the program that created the file, use that program to open the file and read its contents.