The document viewer counts all time the document is left open in a browser window (or tab). Leaving tabs open (as reminders) is a fairly common practice. Being aware of this sort of behavior when tracking can help lead to better design for tracked documents.
Recommendations --
-- maybe use multiple pages, so more granular focus may be tracked accurately.
-- maybe use a cover page, so scrolling action can be contrasted against skims (similar to bounces on a webpage).
-- maybe prioritize MULTIPLE views by a visitor, as opposed to one long single view.