PowerPoint for Mac 2011 doesn't retain the vector format of a placed PDF file, it just converts the vector graphics to low-resolution bitmap graphics when the presentation is saved. This can be avoided by using a .WMF or .EMF file Windows Metafile, instead of PDF. WMF is Microsoft's own (old) portable graphics format which can contain bitmap elements and vector elements.
But the downside of the WMF/EMF format is that graphics with complex effects from PDF-based applications like Illustrator may be rendered wrong, which then makes it necessary to convert the vector graphics into bitmap graphics, after all...
However, 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 or EMF file exported directly from the software used to create the graphics in the first place. For OS X, there are commercially sold conversion utilities and free online conversion services on the Web.
Using vector graphics applications for export or conversion:
• Adobe Illustrator can export to WMF and EMF 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-resolution image yourself, e.g., by using the Preview application to open and export the PDF as PNG. (Beware: Depending on the source of the PDF file, it may contain low-resolution bitmap graphics, which can not be 'upscaled' without visible pixelation.)
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.