I don’t think it’s true in general, you may just used bad PDF viewers. I can scroll through a content-rich PDF much faster and smoother, than a similarly resource-packed website.
Yep. With PDF Expert on an iPad, you can instantly see a thumbnail view of your pages, as well as swipe the page slider through 600 pages with a thumbnail being shown for each of those as you swipe.
This does get slower for some pathological PDFs, which is why an even faster CPU would help.
Yep. You can actually use WebKit to render PDFs in Linux and it works perfectly fine as well. Performance problems seem to be most prevalent when you browse them in a web browser.
