The pictures were compressed using some method that the iPad does not understand, so that they don't open in the document. Computer's generally have much larger libraries of decompression algorithms (because theres plenty of room to support even rare methods.
By opening the PDF in another program and forcing it to save the PDF again using it's own compression software you can make the pictures available on the iPad again. Assuming it uses the more common algorithms, which most free and cheap software does.
Despite PDF being an open standard, some implementations are more standard than others.
Using Preview on an iMac works especially well, since Apple is more likely to support their own subset of the standards. The only problem is that Preview currently has two compression methods. One does little, and the file was 35MB (as opposed to the original 6MB). The second is full on, and while the file got smaller (less than 1MB), the quality of the pictures went way down; notice how bad it looks the moment you zoom in a bit. You can even see some artifacts with out zooming. The original pictures were much higher quality.