Is there an iPad app that will 'play' a large PDF file as a slideshow?
Presentations are meant to be presentations. If you're going to do the process that 3rdparty suggested, I highly advise that you convert the PDF into a presentation friendly format (jpgs are fine if you want to "fudge" it), but invest in Keynote for the iPad and use it!

Keynote iPad's presentation skills are really astounding. Digital Laser Pointer, private presenter's notes on the iPad and the external display you connect to displays the full screen presentation (requires the VGA video out cable or some equivalent products, the composite/component video cables might work too). Transitions, animations, rich media, everything.

It sucks that you already have it in a PDF, but if you do the work to convert it, the iPad will help you present it wonderfully.