According to the Lewis definition of , a compound isn't acidic or basic until it does something. In other words, you can't just say that "X compound is a Lewis acid" unless you've seen it act as an acid in some chemical reaction - in which case you'd say that "X compound is a Lewis acid in this particular reaction." For ##PCl_5## to be a Lewis acid, it would have to react in such a way that it accepted a lone pair from some other atom or molecule (from a Lewis base). It turns out that phosphorus very readily accepts electrons from other molecules, which is why it generally behaves as a Lewis acid.