Create a bookmark for the PDF viewer (e.g. Adobe Reader). When the user clicks on a bookmark, the PDF viewer jumps to that place in the document.
1 is the top level, 2 is the next level, etc.
If yes, the child elements are shown. If no, the child elements are hidden.
Title of the bookmark
<Bookmark level="1" select="$title" open="no" />
Create a bookmark on level 1 (top level) with the title stored in the variable
Chapter about bookmarks.