PDFs get large because of the images inside them: scans, photos and exported slides. Shrinking those images is what makes the file small.
Pick a level
Balanced suits most documents. Use Light if it will be printed, Strong when it only needs to be readable on screen or must fit a strict limit. A grayscale option saves more on colour scans.
The trade-off
Pages are re-saved as images, so text in the compressed PDF can no longer be selected or searched. Keep the original if you need that.
Still too big?
Split it into parts and send them separately.