I think it might have found what was causing the issue. I had made the pie charts transparent. When I removed the transparency, it rendered fine and at high resolution. Then I applied transparency in Illustrator.
I have this problem too. I have a large wall map (42in x 42in) displaying quite a bit of information and I am using pie chart symbology with label expressions. At a scale of 1:300,000 and symbol size of 48 they were exporting fine as pdf. When I made a second map at a scale of 1:700,000 and symbol size of 55, the pie charts no longer display in the exported pdf. Every other symbology displays fine (and this is a busy map!). The dpi is set to 150, CYMK and all that. Still tweaking it to see if the symbol point size has a threshold...
Like I said, the two maps are identical in page size and setup. One has a smaller scale than the other with a handful of more polygons but less line work. The pie chart symbol is the only one that is disappearing upon export. There is no transparency on the pie charts...