My pdf presentation pages show as different sizes

I need some help with my pdf presentation.

Ive put together a presentation using different PDFs I have made from various other presentations and individual PDFs I have converted from a variety of jpgs, Photoshop and excel docs. when I save and then open to view the full presentation, each page opens as a different size, so some fit perfectly into the screen and others are too big, I cant send it to my client like this, what can I do to the pages so they all open the same size?!