Here is a more detailed procedure than what I wrote earlier. It was devised before Save as … disappeared and then returned, so anyone using Lion or Mountain Lion will have to "translate."
To achieve 300 dpi resolution in drop shadows, open the ColorSync utility Filters and duplicate the Create Generic PDFX-3 Document filter. In the copy, click on the arrow to the left to open the drop-down. Then click on the next arrow, Create PDF/X-3 Document. Enter 300 in each of the boxes labeled Resolution. Save it with a new name.
When you want a 300 resolution PDF, go Print > PDF > Open in Preview > Save as… > Quartz > New Name filter > Save. (Do not use Export.)
Using this filter will give you 300 dpi resolution in drop shadows and other transparencies.
Walt