Documents where the viewer can adjust features such as size colour font easily I'm currently working on an assignment for teaching and learning strategies for students with a very broad range of disabilities that can inhibit reading literacy abilities. I'm thinking of an interactive PDF with audio to accompany text. However, because of the nature of my content I would also like the document to be presented in a format where the viewer can easily adjust font size, background colour, typeface in order to best suit their own visual needs. but I have no idea how to do this? any advice?
It sounds like you want to create something in the EPUB format using iBooks Author, Pages, or another app.  You can see how these allow the kind of flexibility regarding typeface/color/font size that you seek by just viewing one in the iBooks app.