I looked at a few technical books I have and none of them are in 8.5x11 or A4 sizes. Most books I have are much smaller than that, and more easily handled.
PDF spec actually supports different page sizes (crop box, trim box, bleed box, media box, etc) for different pages. But it seems like what you what is essentially two separate PDFs with different page sizes combined together, and the reader picks one based on the viewport. I'm not sure PDF spec natively supports that, but PDF does support JavaScript and I can imagine JavaScript querying the viewport and then hiding/showing pages.
PDF spec actually supports different page sizes (crop box, trim box, bleed box, media box, etc) for different pages. But it seems like what you what is essentially two separate PDFs with different page sizes combined together, and the reader picks one based on the viewport. I'm not sure PDF spec natively supports that, but PDF does support JavaScript and I can imagine JavaScript querying the viewport and then hiding/showing pages.