In the reaction
$PC{l}_{5(g)}\rightleftharpoons PC{l}_{3(g)}+C{l}_{2(g)}$
There is 1 mole of gaseous species $\left(PC{l}_5\right)$ on the left and 2 moles of gaseous species ( $1PC{l}_3$ and $1C{l}_2$ ) on the right. That is the no of gaseous species increases from left to right and vice versa. We know that an increase in pressure (decrease in volume) favours the net reaction that decreases the total no of moles of gases (the reverse $r\times n$ ) leading to an increased yield of $PC{l}_5$.