Some driftwood has tannins (tannic acid) that will lower the pH of neutral water. However, plywood should not be leaching into an aquarium. If it's leaching, I'd be concerned it was some chemical applied to the plywood. Plywood itself is typically light colored, which, iirc, is an indicator or low tannin. However, I'm not a wood specialist, so...
Is the plywood soaking up water and dripping into the tank? If so, you could collect and test some of the drippings.