Sometimes when I export a map to PDF, some of the points are solid black instead of the intended colour. It doesn't always happen, there doesn't seem to be any reason I can detect. One day it happens, the next it doesn't (to the same mxd document) Sometimes if I increase the dpi to 400, 500, 600 etc. the problem goes away, but this isn't a practical solution.
Michael,
Issues crop-up with exporting to PDF from time-to-time - I can remember all sorts of fun stuff happening years back, like fire hydrants turning into hurricane symbols on export, etc.
I would recommend looking over 17783 - Diagnose an ArcMap printing or exporting problem
There's some info on troubleshooting the export and some recommendations on how to remedy.
This is specific to exporting to PDF, it never happens when exporting to JPG.
I don't know if this will help ... but check to see if you have the font for the symbols being used on the machine you are printing from. I am assuming that you have some vector or esri fonts in your graphic.
I had similar problems and the relate to font substitutions being made when the pdf conversion cannot find them on the machine the document is being printed from.