9. As shown in Figure 2, in rectangle $A B C D$, $A B=7, A D$ $=24, P$ is a moving point on side $B C$, and 
$P E \perp A C$ at point $E$, 
$P F \perp B D$ at point $F$. 
Then $P E+P F=$ $\qquad$
9.6.72.

Let the intersection point of diagonals $AC$ and $BD$ be $O$. Connect $OP$.
From $AB=7, AD=24$, we get $OB=OC=\frac{25}{2}$.
From $S_{\triangle OBC}=S_{\triangle OPB}+S_{\triangle OPC}$, we get $\frac{1}{2} \times \frac{1}{2} AB \cdot BC=\frac{1}{2} OB \cdot PF + \frac{1}{2} OC \cdot PE$,
which means $PE + PF = \frac{AB \cdot BC}{2 OB} = \frac{7 \times 24}{25} = 6.72$.