Any "actual" acid requires hydrogen and I'm uncertain of whether or not wood has hydrogen but would assume so knowing that it ignites the oxygen around it so it could be considered an acid/Ph decreaser but is different to plants than water or us. Low Ph still can affect plants but otherwise sawdust Always works so think of it as sawdust with some oxygen.
Wood ash is alkaline. It's how lye is made for old time soaps (by burning hardwoods and seeping the ashes). Very alkaline. It can be used to modify the pH in soil but be VERY careful because a little goes a long way and can quickly make the soil unusable. Plants require an almost neutral pH to grow, and soils are often acidic. It's best to get soil tested first. Also there are better ways to decrease the acidity in soil such as adding organic matter.