My brother-in-law has many oak trees in his front yard and every fall he rakes up the leaves. I told him to just mulch them in (the soil is quite sandy) as they will help improve the soil. He said that would make the soil too acid. Will this dramatically raise the soil acidity? What if he composted all the leaves; would the compost be too acid to use as well?

Actually, your brother-in-law has a gold mine. Oak leaves in particular are very good for garden soil as they contain micronutrients drawn from the depths of the earth. The best way to add them into the garden is to rake them into a shredder or run over them with a power mower. The oak leaf needs to be broken up in order to release its nutrients. Then they can be tilled into the garden bed using a garden fork. If left intact, oak leaves mat together and promote unhealthy bacteria, which can be transmitted to planted material. Once shredded, they can be worked into the sandy soil, offering an excellent organic amendment to a soil that drains too quickly. It will not dramatically raise the soil acidity. The leaves can be composted but again, must be shredded or chopped up to effectively decompose. If mixed in with ongoing compost, they will degrade much faster and more thoroughly.