pg is a program designed for use with a technology no longer in much use: a bare CRT/keyboard combination connected over a communication line (modem, serial line, or similar) without any computer at the terminal end of the line. It performs functions similar to less.
The command is affected by environment variables COLUMNS, LANG, LINES, SHELL and TERM as described in the environ(7) man page.
Most (all?) Linux distributions incorporate this from the [util-linux] project