The buffer is made of a weak acid and a salt, I would therefore think it's an acidic buffer already. The colour of Phenolphthalein confirms this as the indicator is colourless in an acidic solution. My guess would be that the question is asking for you to suggest a pH corresponding to an acidic solution (less than 7). But don't put too low a number, like 1/2 since a buffer shouldn't be a very strong acid.