That link confirms what I thought. iPhoto is for images. A PDF is not an image. Therefore, iPhoto can't import it. I
have seen these 'posters' made on a Mac but always assumed they were made by dumping the individual images (all the same size, of course - either because they are or by using a script/Automator) into a canvas lare enough to hold them (20 x 30 inches).
Then the image can be imported/seen by iPhoto and its rather expensive Book/Printing connections can be used.
I'd give the Kinko shop a try first.
And there are probably several locla print shops that could print the PDF, also.