Three levels
- Light: images at print quality. The file shrinks a little and looks the same.
- Recommended: images at screen quality. The best balance for most documents.
- Strong: the smallest file, for when the limit is tight.
What gets smaller
Compression mostly reduces the images of a PDF: scans and photos. A PDF of plain text is already small; if compressing would not make it smaller, you get your original file back.
