Lossy vs lossless compression, explained
Every compression tool uses one of two strategies — lossy or lossless. Understanding the difference helps you pick the right setting and avoid ruining a file.
Lossless compression
Lossless compression makes a file smaller while keeping every bit of the original data. Decompress it and you get back a perfect copy. PNG, ZIP and FLAC are lossless. The trade-off: the savings are modest — typically 10-50% for images.
Use lossless when you cannot afford any quality change: legal documents, archival masters, line-art graphics, or anything you'll edit repeatedly.
Lossy compression
Lossy compression achieves much smaller files by discarding data the human eye or ear barely perceives. JPG, WebP (lossy mode) and MP3 are lossy. You can shrink a photo 70-90% with no visible difference — but the discarded detail is gone for good, and re-compressing repeatedly degrades quality.
Use lossy for photos, web images, and anything where small visual changes don't matter.
The sweet spot
For most web images, lossy at 78-85% quality is the sweet spot — visually identical to the original at a fraction of the size. Below ~70% you start to see artifacts.
What about PDFs?
PDF compression is usually lossy when the PDF contains images or scans: the pages are re-rendered as compressed JPEGs at a lower DPI. That's how Shrink44's PDF compressor cuts 60-85% off scanned documents.
Try it
Open the image compressor, slide quality from 100% down, and watch the file size drop while the preview stays sharp — that's lossy compression finding the point where smaller stops being visible. Keep an original copy if you'll need lossless later.