What were your export settings?
Did you try the PDF optimizer in Acrobat?
And what's the intended use?
PDF settings where Smallest File Size default settings.
I did run the pdf through optimizer with no real change.
Downsampling was set to 72
When I change Image Quality to LOW some of the photos where very pixelated.
The intended use is so the customer can send out bulk emails to potential sponsors so keeping the size down was what they asked for.
But every time I edited the photos down the photos started looking bad and there was no point sending that out.
Copying all photos and transparencies into photoshop kept thing looking sharp but was time consuming to do.
Smallest file size downsamples to 100ppi