(I'm taking back my previous answer.)
PowerPoint for Mac 2011 doesn't retain the vector format of a placed PDF file, it just converts it to low-resolution bitmap graphics when the presentation is saved. This can be avoided by using a .WMF file, Windows Metafile, which is Microsoft's own (old) portable graphics format that can contain bitmap elements and vector elements.
So, to keep vector graphics in vector format within PP, the PDF file will need to be converted to a WMF file, although it would be preferable to get a WMF copy exported directly from the software used to create the graphics in the first place. For OS X, there are commercially sold conversion tools and also free online services.
Using vector graphics applications for export or conversion:
• Adobe Illustrator can export to the WMF format, so if AI is at hand, open the PDF file in AI and choose File > Export…, then choose "Windows Metafile wmf" from the format selection in the dialog box.
• Inkscape (open source software) can also export to the WMF format (File > Save As…), and placing the WMF files created in Inkscape seems to work. But when it comes to importing and exporting existing PDFs, Inkscape is fickle and may throw some error message, with the resulting file not working in PP.
Wikipedia has a comparison of vector graphics applications with their export formats.
If this is all too much trouble and you want to keep at least a high-resolution bitmap image, the best workaround is to convert the PDF into a high-ppi image yourself, e.g., by opening and exporting the PDF as PNG or TIFF in Preview or from the original creator software.
After placing the image, it's important to adjust PowerPoint's automatic image compression setting before you save the presentation: Choose File > Reduce File Size, and in the dialog box select "Keep Current Resolution"from the drop-down menu.