Improve documentation: a response with a pdf file needs schema format 'byte' to get a byte array

I could be mistaken, but I believe your original thinking was correct.

The specification clearly says that `format: byte` is for base64 encoded data, https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.3.md#data-types. Which i personally wouldn't interpret as byte[] in Java. Something is fishy here.

But maybe frantuma knows more.