The 2019 PGA Championship cut line is determined by reducing the 156-player field down significantly heading into the final two rounds.
The 2019 PGA Championship cut rule is similar to what you'll see week to week on the PGA Tour, with the top 70 players and ties getting into the final two rounds. There is no 10-shot rule, which means there's no guarantee that a player within 10 shots of the lead qualifies for the weekend rounds. There is a 10-shot rule at the Masters.
There's also no secondary cut if 78 or more professionals make the cut, as would be the case in a standard PGA Tour event.
The cut line is hovering between 3 over par and 4 over par, with the 20 PGA of America club professionals subject to the same cut as the other players in the field.
Typically, players must be within five shots of the lead heading into the weekend to have a reasonable chance to win. With Brooks Koepka so far in charge of the tournament, that might even be too much to ask of the rest of the field.