This property represents the "Limits" entry of the number tree
node dictionary object.
It is defined as part of the PDF 1.0 specification.
It is an Array containing two integer representing the lower and
upper bounds of the tree.
The first int represents the lowest key in the EntryNums entry of this or any child
The second int represents the highest key in the EntryNums entry of this or any child
For definitive details see:.
The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 37,