The majority of cats aren't any particular breed and aren't even a mix of breeds. Without papers, there's no way to tell. Having said that, though, it looks like she might have a bit of a smooshed face. There are a few breeds with that characteristic, such as Persian, British Longhair/Shorthair, and some others, and it doesn't seem to be a common characteristic otherwise, so perhaps there's a little bit of purebred in her history somewhere. Who knows; there's really know way to know for sure, but it's fun to imagine, isn't it?