Take a PDF that is formatted for a 8.5 x 11, then try to read it on a 4 x 5 screen. It just doesn't work. Word docs, RTF files, and other structured doc formats (e.g. HTML) can be re-flowed to fit the screen correctly, but a PDF file needs to use the "tagged" PDF format and have the structure tags included in the doc or else it will not be able to adjust to the different display size.
