Bi means someone finds male and female genders attractive (masculine or feminine). They probably aren’t attracted to androgyny or less “traditional” genders (since it becomes pan).

Pan means gender doesn’t matter. Enbies, trans people, and anyone else could be attractive.

In other words, Pan is Bi++

If we aren’t splitting hairs, they are mostly used synonymously, but “pan” is more precise for folks that open to partners regardless of their gender.