Something strange is going on there. The text seems to be black, but it's defined as a spot color. Also, the page number at the lower side of the page seems to be a form (?) and has some transparency around. These mistakes alone might make the PDF larger. Try solving the above issues and make a test to see if the size decreased.
In Acrobat, click the File menu and choose Save As Other > Optimized PDF. In the PDF Optimizer window that appears, enable only the "Discard User Data" checkbox, then within the Discard User Data settings, enable only the "Document Information and Metadata" checkbox and save the PDF. On my computer, the PDF went from 13MB to 20K.
Sustainable Printing Goes
Far Beyond Using FSC Certified
or Recycled Paper
This informative paper on deinking: demand, principles, problems and solutions also explains why printing technologies are not all equally compatible with paper recycling systems; and why just a small fraction of printed material in the paper can cause difficulties. Link To White Paper