A large PDF is usually caused by high-resolution images, scanned pages, embedded fonts or unnecessary document data. The best way to reduce size depends on what the PDF contains and how it will be used.
A PDF made from photographs can become very large because each page may contain a full-resolution image. For documents intended only for screen viewing, extremely high image resolution often provides little visible benefit. Printed brochures, technical drawings and photographs may require more detail, so aggressive compression is not always appropriate.
Scanning every page in full colour at very high resolution can create a huge file. For ordinary text documents, grayscale or an appropriate black-and-white scan may be sufficient. Always keep readability as the priority: signatures, small text, stamps and diagrams should remain clear.
Compression is a trade-off. Reducing image quality can make a PDF much smaller, but repeated compression can introduce blurry text and visible image artifacts. Keep an original copy before experimenting with stronger compression.
Compare the compressed document with the original at normal viewing size and at zoom. Pay special attention to fine print, tables and images containing text. Also confirm that hyperlinks, selectable text and form fields still work when they are important to the document.