Best format for digital document with lots of images

So, on InDesign I created a this historical information document which includes many small images, some of which have links to websites (references). It is too large and I don't know what to do about it.
My hope was that this document would be small enough to load quickly on an iPad but at the same time that the images would be high-quality enough that I would be able to zoom in and not get a very pixelated image.
I converted most of the images from png to vector. This took my document from about 12 mb to 45 mb and there are still like 20 images I have not converted. After vectorizing the images, I made them very small, about 1.5 in, and I simplified and flattened their transparency. I don't know what else to do.
When I export the file as a PDF and open it on my iPad, it takes forever to load. I have tried downsizing the PDF, that takes me from 45 to 42. Any suggestions?