IME driftwood has done very little to lower pH, especially in harder and high pH water. While the tanic acid (tannis) in aquarium driftwood will assist in lowering pH, the driftwood alone wont lower pH. If it does it would hardly be noticeable. If it did, the effects would only be tempoary until the majority of tannis leeces out. Douglass fir and cedar would not make good driftwood for an aquarium. In fact it could be harmful. It would more then likely rot and mold. The decaying wood would produce ammonia which can overload a bio-filter. If you want to try driftwood I reccomend aquarium suitable driftwood or "real" driftwood. Driftwood is usually teak or similar bog woods (trees). Usually it is imported from Thailand and Malaysia etc. Driftwood is best used for decor, hiding spots for fish, adding a tea color look for Black water fish. It will also help keep the water soft for a certain ammount of time.
To lower pH use CO2 injection or RO/DI or distilled water with freshwater tropical fish trace element supplements. On the other hand don't worry too much about pH or keep fish that like a high pH like Brakish Water, African Rift Lake Cichlids or Salt Water fish.