Creating "intra-document" hyperlinks out of the text on the page & an automatic index generator.
I have a few repair manuals for vehicles (1300+ pages each) and I use ZODO because it loads big pdf files fast and the "find in document" is faster than others. But one thing would make my experience better and that would be a feature to let me select text and turn in into a hyperlink to a page within the document. An even BETTER feature is to use "find in page" in conjunction with hyperlinking so that, for example, FIND ALL "Alternator" →CREATE LINK TO "PAGE 11-34." That way all instances of the word alternator will lead me to the specific page about it.
Also most text books have indexes in the back. It would be nice to be able to make my own for these manuals.