PDF compression is really two different technologies wearing one name — and knowing which one your file needs is the whole game.
A PDF is a container of objects: fonts, images, page trees, metadata. Tools like Kameleo's Balanced level rewrite that container more efficiently — object streams, deduplicated resources, stripped junk — without touching a single pixel. Text stays selectable, quality is identical. On a already-optimized digital PDF this may only save a few percent; on a bloated export from Word or a design tool it can cut half the size.
Scanned documents are just photographs of paper. The only way to make them dramatically smaller is to re-encode those photographs — lower the resolution to what reading actually requires and use stronger JPEG compression. Kameleo's Strong and Maximum levels do exactly this, page by page, on your device. Savings of 60–90% are normal on scans. The trade-off is honest and unavoidable: pages become images, so text is no longer selectable.
Digital PDF you'll keep editing or need text-search on: Balanced. Scanned contract you need to email under a size limit: Strong. Archiving thousands of old scans: Maximum. If Balanced barely moves the number, that's your signal the file is image-heavy — switch to Strong.
Most online compressors upload your document to their servers. Kameleo compresses it inside your browser — the file never leaves your device, and you can download a privacy receipt with SHA-256 hashes computed locally as proof.