Are you confusing rescale as a change of resolution and compression as a change of filesize?
You can not compress JPG files as much, because the data is already pretty much compressed. You can downscale the DPI resolution or the resolution of the picture, making it smaller size, but you lose image quality (which you actually lose when you start working with JPGs). You can use almost any image viewer, it is even possible online.
When compressing PDF files, the results will vary according to what is inside the PDF. I think vector drawing will shrink fairly well, not so much pictures...