The “inner glow” is an affect style created directly in InDesign CS4. It is part of a box style. This box style is used a number of times in the 30 page document. The glow is a gradient from black to white which looks like an inner shadow for the boxes.
When I alter the style in InDesign to say something simple like a 10% black solid fill, the pdf file size decreases to about 1 Mgb.