When compression works, and when it doesn’t
PDF size is almost always dominated by images. A scanned PDF, a photo-filled brochure or a document with high-resolution images can shrink a lot. A PDF that is mostly text and vector graphics is already small, and there is almost nothing to save because there are no images.
That is why this tool shows the size before and after, and does not offer a download when the result is not smaller than the original file.