Three modes for different documents
Image compression is a sensible starting point for PDFs with photos or scans. Lossless mode rewrites the file structure without deliberately reducing image quality. The strongest mode re-renders pages as images and offers resolution, JPEG quality and grayscale controls.
When smaller means a tradeoff
Lower image quality or resolution can blur fine print, diagrams and photographs. The strongest mode changes page structure and can affect forms, links and accessibility. It can preserve recognized text as an invisible layer, but review search and copy behavior on your output.
Why some files barely shrink
A PDF that already contains efficient images or mostly text may offer little room for reduction. A rewritten file can even be larger. There is no promised reduction percentage or fixed target size; choose settings based on your actual document and the recipient’s requirements.