I don't know what he's talking about. The only thing about having an Expandable PDF is having expandable text fields. It has nothing to do with the images and all forms of PDFs can be zoomed in and out of. He should also never try to enlarge an image larger than the size you give him. Enlarging them would only decrease the quality. Make your images as large as you want them printed, tell him what size they are to be printed at, and put them either all in one PDF as separate pages, or save them as individual PDFs.