取BD的中点为F.
∵AB＝AD、F∈BD且BF＝DF,∴AF⊥BD,又CD⊥BD,∴AF∥CD,
∴∠PAF＝PA、CD所成的角.
∵梯形ABCD是直角梯形,又CD⊥BD、AD∥BC,∴AB⊥BC、AB⊥AD.
∵AB⊥AD、AB＝AD＝3,∴BD＝3√2.
∵AB⊥AD、BF＝DF,∴AF＝BD/2＝3√2/2.
∵PB⊥平面ABCD,∴AB⊥PB,又AB＝PB＝3,∴PA＝3√2,∴PA＝2BD.
∵PB⊥平面ABCD,∴AF⊥PB,又AF⊥BD、PB∩BD＝B,∴AF⊥平面PBD,∴AF⊥PF.
由AF⊥PF、PA＝2BD,得：∠PAF＝60°.
∴PA、CD所成的角为60°.