Hi, I agree with apepp and add this:
jpeg compression affects the image (with more or less damages), LZH compression affects the file only, but some softwares cannot read it
when you flatten your file, everything on it is "congealed", for example texts don't rely on fonts installed but are just pixels; same for the style, aso
in some cases, when you flatten your image some changes may occur. In this case, you can try <Merge visible> as a first step
Hope it helps, and good job