As Bill Cody would say, "It depends." Here's how it works. If the water of your pond is buffered, it will absorb acid from decaying leaves.
"Buffered" basically means how much lime (calcium carbonate) is dissolved in the water. Lime normally comes from soils. Some soils have it, some don't. For example, sand doesn't. Most clay does. Sometimes, there are limestone deposit beneath subsoils. If your water has 150 ppm calcium carbonate in it, it can buffer 150 ppm of acid.
When leaves fall into a pond, they steep, like tea. The brown of leaves is made of tannins...tannic acid.
So, if your pond has very little buffering capacity, it can (and will) be absorbed by the tannic acid of leaves.
If you want to know how the leaves will affect your water, have the water analyzed.
