Shrinking a PDF without making it unpleasant to read
Compression is mostly a trade-off between file size and the quality of the material inside the PDF. A document made from high-resolution scans can often be reduced dramatically, while a mostly text-based PDF may already be close to its practical minimum. The right setting depends on what the file is for: email attachments, online forms, archive copies, presentation handouts, and print-ready files do not all need the same level of detail.
Start with the least aggressive reduction that solves the problem. Fine text, diagrams, signatures, and screenshots can become hard to read long before a photograph looks obviously degraded. If you are optimizing several similar files, test one representative document first, then compare the compressed copy with the original at normal reading size and at a closer zoom.