P is a moving point outside the rectangular ABCD, and satisfies the AP vertical CP BP.DP When point P changes, does the degree of angle BPD change? If not, calculate the degree of angle BPD; if not, calculate the range of change

P is a moving point outside the rectangular ABCD, and satisfies the AP vertical CP BP.DP When point P changes, does the degree of angle BPD change? If not, calculate the degree of angle BPD; if not, calculate the range of change

Abcdp five point common circle
BPD = 90 degrees