This manual page is not meant to be exhaustive. The complete documentation for this version of TEX can be found in the info file or manual Web2C: A TeX implementation.
The pktype program translates a packed font file (pk) (output by, for example, gftopk(1) to a file that humans can read. It also serves as a pk file-validating program (i.e., if pktype can read it, it's correct) and as an example of a pk-reading program for other software that wants to read pk files.
The pk_file on the command line must be complete. Because the resolution is part of the extension, it would not make sense to append a default extension as is done with TEX or DVI-reading software. If no output_file is specified, the plain text translation is written to standard output.
pktype looks for pk_file using the environment variable PKFONTS. If that is not set, it uses the variable TEXPKS. If that is not set, it uses TEXFONTS. If that is not set, it uses the system default.