Why PDFs can shrink so much
A PDF isn't really one file format. It's more like a container that bundles text, fonts, vector graphics, and raster images together. So when people say "compress this PDF," what they actually mean, almost always, is shrink the images inside it — in a typical scanned document or photo-heavy PDF, those embedded images can easily account for 90%+ of the total size.
That's also why two 10-page PDFs can be wildly different sizes. One built from clean vector text compresses down to almost nothing. One built from full-resolution photos can run into dozens of megabytes, even at the same page count.
Where the size actually comes from
Every image inside a PDF gets stored with some encoding, usually JPEG for photos or a PNG-style lossless scheme for graphics and scans. Two things control the size from there: resolution (how many pixels) and encoding quality. A photo saved at 300 DPI for print is way bigger than the same photo at 96 DPI for a screen, even if they look identical on your laptop.
That's the first thing any half-decent compression tool does: shrink the resolution down to whatever the PDF actually needs before re-encoding, rather than just cranking up the compression on an oversized image.
Lossy vs lossless compression
Lossy compression, standard JPEG being the obvious example, throws away detail the human eye barely notices anyway: subtle gradients, fine high-frequency texture. Push it too hard, though, and you start seeing it — blocky artifacts, smeared edges, text that looks slightly melted. That's why an over-compressed scanned document looks rough.
Lossless compression doesn't throw anything away. Every pixel comes back exactly as it went in, just packed more efficiently. Good for text, line art, anything with transparency. The tradeoff is it just can't shrink a file as much as lossy compression can, full stop.
The practical version
If your PDF is mostly photos and you just need it small enough to email, moderate JPEG compression around 80-90% quality usually cuts the size way down with zero visible difference. If it's scanned text you need to stay legible, keep the resolution modest instead of hammering it with lossy compression — a clean scan at 150-200 DPI shrinks fine on its own without turning into mush.