Why PDFs get so large
A ten-page scanned contract can easily reach 25 MB because each page is stored as a full-resolution photograph. Digital PDFs bloat for different reasons: embedded font subsets, duplicated logos on every page and uncompressed screenshots.
Compression targets exactly that overhead. It re-encodes the images at a sensible resolution for screen reading and removes duplicated resources, which is why the text of the document stays crisp while the file gets several times smaller.