A 20 MB PDF won't fit in most email inboxes, and slow to upload anywhere. The good news: most PDFs are far larger than they need to be, and you can usually cut their size by 50–90% without any visible loss in quality.
Where the size actually comes from
Two things dominate PDF size: embedded images and embedded fonts. Scanned documents and photo-heavy decks are mostly images, so re-encoding those images at a sensible resolution is the biggest win. Text-heavy documents (reports, tutorials, contracts) are mostly fonts and text streams — there, subsetting the embedded fonts to only the characters you actually use can cut the file dramatically.
Picking a compression level
Use Maximum when the file is just for email or screen viewing — it downsamples images hardest. Use Recommended for a balance of size and quality, which works for most documents. Use Light when you need to keep print-quality images and only want to trim the fat.
With EditJ's Compress tool, you'll see the true before/after size and the exact percentage saved, so there's no guessing.
Tips
Compress as the last step, after you've finished editing. Re-compressing an already-compressed file gives little extra benefit. And if a document is purely text, expect a smaller percentage — there's simply less to squeeze.