Keeping history of visited location (like temporary bookmarks), with icons/commands : [keep][previous][next][clear] == [!][<][>][x] to navigate back and forth part of the document, like "a few fingers" can keep pages "instantly accessible" while working inside a "real" book.

A list-box (like the bookmarks list box) would be convenient, with selected text as a label for a "kept location".

This should be an "application, per document" feature, with [keep] locations stored in an "ini-like" file, not to pollute the document's proper "bookmarks and annotation". With the [keep] memorized location stored through time, it is a memory of the "work session" inside the documents.

It could then be "quite easy" to convert "kept locations" into permanent "annotation" inside the pdf ... if needed.