The biggest trick is in both image compression and quality. Most PDFs don't need huge dpi resolution quality for its images if you're only meant for on screen or printing for legibility only. It's only when you're creating something for a printing shop or similar that the images get unwieldy, or when producers put in things like backgrounds or watermarking on every page. paizo learned a similar lesson about three or four years ago, and for their major releases have both "full" and "lite" versions for download.