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Copy the image, then paste it into powerpoint slide . Next, save the mage with PDF formate it will convert to PDF.
Assuming you have Adobe CS, I'd rather use Photoshop: save as -> pdf
Yet, this method will produce a quite huge filesize.
Alternatively you might want to save it as EPS and convert it afterwards with Distiller to PDF.
Another alternative you might want to consider is Illustrator or inDesign to export, respectively print as PDF.