As Nathan Yegler already blogged: “it’s more fun to write programs to help you write slides than it is to write slides.”. There is a plethora of programs out there that allow you to make slides in many forms and there are books on the subject as well¹.
From personal experience I can recommend starting with a structured mark-up based format for editing that is then converted to PDF and your preferred ebook format (e.g. EPUB). Using mark-up instead of visual positioning will make it easier to keep the slides consistent. Sphinx, based on the reStructured Text markup, can generate both PDF and EPUB and an extension allows you to embed your slides on
If your slides are going to contain mathematics formula, you might want to look at LaTeX and the slide generation package (beamer) for that instead.
¹ Edward Tufte has written a great essay on the problems with PowerPoint presentations, a must read for anyone making slides.