Mac becomes slow and unresponsive when opening complex pdf
I ran into some issues like this with a PDF which contained some embedded vector images… the images were plots generated by Mathematica which had been exported as PDF files. The images were composed of a few thousand data points. As Preview attempted to plot these (thousands of) vectors, it would lock up (just as you describe). 

Exporting the images from Mathematica as a PNG (raster format) solved the problem instantly. I'm not very familiar with the behavior of PowerPoint with vector images (if I recall correctly, the two do not mix…). Could you try rasterizing some of the more complex images prior to placing them in the document? 

Note: as in your case, the PDF file didn't seem particularly large (the file size of the PDF image compared to the PNG image was very comparable), but the amount of processing power required to render the PDF brought the system to a crawl every time I scrolled to the point in the document where the complex plot was located.