Demolishing a shed floor made of plywood. Can I use circular saw?
A circular saw can be used to plunge cut into flooring cut out sections for repair/access.  I don't see how this would be any different.  Just make sure you adjust the blade depth for the width of the plywood.so that you don't hit anything that might be under the shed (support beams/metal/concrete/small animals too stupid to move/etc).