A file with a .pfx extension is a system file, typically containing encrypted information. This type of file can only be read by certain programs.
Files with the .pfx extension store an SPC, or Software Publisher Certificate, and use encryption to determine who has access to system programs or even the system itself. PFX files may be opened using Adobe Acrobat or Acrobat Reader using a password. These features make PFX files part of the security measures used to prevent unauthorized access to a computer's files, programs and systems.
Not all authorities that issue SPCs use PFX files. Some use Private Key, or PVK, files to store the private key and then store the data in either an SPC or CER file.