OR
The document is the result of scanning on some budget all in one scanner/printer/fax with built in pdf software.
See my #179.
If you scan from the scanner then the scanner software makes the most detailed image it can depending upon the presets, but if you scan from a full version of Adobe Standard where Adobe controls the scanner, then the Adobe Standard presets control the entire condensation process that makes a pdf much smaller than a full jpg or tif scan from the same scanner.
See above #179.