Compression

Compression makes a PDF smaller without changing what it looks like, or at least without changing it noticeably. PDFs balloon in size mainly because of images: a report full of high-resolution scans or photos can be tens of megabytes, too big to email and slow to load.

There are two flavors. Lossless compression squeezes the file by removing redundancy and rewriting it more efficiently, with zero change to quality, ideal for text and vector content. Lossy compression goes further by reducing image resolution and re-encoding pictures, which can shrink a file dramatically at the cost of some image detail. Good compression picks the right approach per element.

The practical goal is hitting a size that is easy to send and store while keeping the document readable. Dropping image DPI to a sensible level and re-encoding embedded pictures is usually where the big savings come from.