You will need some application to prepare the graphics -- Keynote works well (I also tried PowerPoint). Select the graphic. Copy it to the clipboard (using Command+C or Edit » Copy).
Then go into Preview, and type Command+N ( or File » Create From Clipboard). Save the new file as a PDF. Notice that, no matter how much you zoom, the image does not pixellate.
[crarko adds: I tested this, and it works as described.]

