The whole document is not loaded at once, document pages are loaded as you are about to view them. This is done via HTTP byte-range requests. So once the conversion is completed (first view), document loading is very fast. The DocumentViewer uses a special web-friendly format (.xpz) and this file contains all vector/raster images and text which is similar to a Pdf file (portable document). Also by default this file is encrypted so only the DocumentViewer can open it.