Most of a PDF's size is images: scans, photos and high-resolution illustrations. Compression downsamples and recompresses them, while text and vector graphics stay as they were, still selectable and searchable.
Pick a level: “Strong” brings images to about 72 dpi for on-screen reading, “Medium” to about 150 dpi, a good balance for email and portals, and “Light” to about 300 dpi for printing.
If a PDF is mostly text, there's little to compress. In that case you get the original back with a note that it's already compact.