What Is a CGI File?
A file with the CGIfile extensionis a Common Gateway Interface script.
These are often seen in a web server’s “cgi-bin” directory.

In this example, changing .CGI to .PDF should let you open it in aPDF viewer.
The same process should work with any file in this context that’s improperly named.
Renaming like this doesn’t actually convert the file to a new format.
It just changes which program opens the file.
Remember what was above about renaming a CGI file.
An actual file conversion takes place with afile converter.