PowerPoint 2011 and 2016 don't retain vector graphics from PDF files, but merrily convert everything into bitmap graphics when the presentation is saved. To embed vector graphics in the presentation file, it's necessary to use an .EMF or .WMF file (Enhanced Metafile / Windows Metafile) instead of PDF. These are Microsoft's own portable graphics formats from the Windows world and can contain bitmap elements and vector elements. The EMF format is preferable over its older version WMF because EMF supports vector graphics better.
To embed vector graphics from a PDF file, the PDF file will need to be converted to an EMF file, but if possible, it would be preferable to use an EMF file exported directly from the software that was used to create the graphics in the first place. Vector graphics editors like Adobe Illustrator and Inkscape as well as OpenOffice Draw all can export .EMF files. (Affinity Designer currently not so.) The possible downside is that more complex graphics and effects from advanced applications like Illustrator may be rendered wrong in the EMF format, which then makes it necessary to convert the vector graphics into bitmap graphics after all.
For conversion, there are commercially sold utilities and free websites. There are also tools to convert PDF files directly to PowerPoint presentations (.pptx files), but I'm not sure about the quality of these solutions, e.g., they might just convert the vector graphics into bitmap graphics...
Using vector graphics applications for export or conversion:
• Adobe Illustrator can export to EMF format. So if AI is at hand, this is the tool of choice: Open the PDF file (or the original .AI file) in Illustrator and choose File > Export…, then choose "Enhanced Metafile (emf)" from the format selection in the dialog box.
• Inkscape (open source software) can export to the EMF format (File > Save As…), and placing the these files in PowerPoint seems to work fine. But when importing and re-exporting existing PDFs with complex graphics, e.g., from Illustrator, the result may differ from the original.
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.