What is the meaning of Pj?

A letter of the Mazahua alphabet.

this

Unmarked for number and gender, but treated syntactically as masculine plurals when used with participles and relative forms, and as feminine singulars when referred to by resumptive pronouns.

Joined by n(j) to nouns they modify.

Used with suffix pronouns.

Originally joined by n(j) to nouns they modify; later without it.

Alternative transliteration of py.

Source: wiktionary.org