What gets smaller
Most of a large PDF is usually its pictures: scans, photos, screenshots. Compression scales each one down to a sensible size and saves it as a JPEG. Light keeps pictures up to 2400 pixels across, plenty for print. Strong goes to 1500 and saves at a lower quality, for the smallest file that still reads well on screen.
What stays the same
Text, fonts and drawings are not touched, so text stays sharp and selectable. A picture is only replaced when that saves real space, and the file is rewritten in PDF's more compact layout.
Questions
Why didn't my PDF get smaller?
If it is mostly text, there is little to shrink: text is already compact. Pictures stored in unusual ways (some print colour spaces, masks made from exact colours) are left alone rather than risk changing how they look.
Is my PDF uploaded?
No. It is compressed in this page, on your device. Nothing is sent anywhere.