Driftwood is generally not pre-soaked and in most cases will release tannins. The amount depends on the type of wood (Driftwood releases more than Spiderwood for example).

Tannins are beneficial for the water environment, but some keepers feel they are unsightly. In such a case, this can be removed with filter material like Seachem Purigen, which will filter any tannins out of the water.

But doesn't it lower the pH?

Yes, tannins can lower the pH of your water. The impact is limited though, and the amount needed to have a meaningful impact would be rather large.