A table of contents whose entries jump to the right page, words that open a web address, and page numbers in the reading software that match the ones printed on the page.
A long document is only useful if a reader can move about in it. A line in a contents page can be made to jump to its chapter, a word can be made to open a web address, and a page can be given the number that is printed on it rather than the one it happens to be.
A document can also say how it wishes to be opened: at a particular page, at a particular magnification, with the contents panel already showing, or as a slideshow that moves on by itself for a screen in a waiting room.
A plain first slide and thirteen more, each stating the effect it arrives by — the twelve the PDF standard (ISO 32000) defines — how long that effect takes and how long the slide stands before the next one comes. The file asks to open full screen, which is where the effects play, and a link on the first slide shows the screen again, brought in by dissolving.
Four pages that say how they want to be opened: which page comes up and how much of the window it fills, the chapters beside it, the pages two at a time, a print dialog already filled in — every one of them a wish reading software answers for itself.
A document made to be followed rather than looked at: every line of the contents lands on the page it names, and every page leads back to where it came from.
Links and marks hung on text the engine has already placed: a band behind a line, an underline, a boxed cell — every rectangle taken from what the layout and the table report afterwards, none of it measured by hand.
Pages numbered by their labels rather than their position.
Two stories over two pages: each one begins in a column of the front page and carries on in a column of the second, and the two swap sides on the way. The file says which column follows which, so reading software goes on with the story it is on.