Oxidation state of P in PCl5 is +5. Since P has five valence electrons in its valence shell, therefore it cannot increase its oxidation state beyond +5 by donating its electrons, therefore PCl5 cannot act as a reducing agent. However, it can decrease its oxidation number from +5 to +3 or some lower value, therefore PCl5 acts as an oxidising agent. For example, it oxidises Ag to AgCl.