Light only repacks the PDF's internal structure — text stays selectable and vectors stay sharp, but savings are usually under 30%. Aggressive renders each page as a JPEG at your chosen DPI/quality — savings of 50–90% are common, but the result is a flat raster (no selectable text).
Web (72 dpi) for online viewing, Email (96 dpi) for sharing, Print (150 dpi) when print quality matters. Use Custom if you need a specific DPI or quality setting.
Light: 10-30%. Aggressive: 50-90%, depending on DPI/quality and how much of the original file was already images.