The CropBox for the page.
The CropBox defines the visible part of the physical medium on
which the page may be displayed or printed. This is what you see as
the boundary of the page when you view a document using Acrobat
These boundaries may be defined directly on the Page object or
they may be inherited from a parent Pages object.
The CropBox is optional and if this property is null it is
implicitly assumed to be the same as the MediaBox.
Assigning a new value to this property will change the property
for the current Page rather than any Pages object from which the
value may have been inherited. In this way the property exhibits a
Attempting to assign a null value to this property will result
in a NullReferenceException being thrown. This is because the
CropBox may be inherited from a parent page and thus removing the
CropBox from the current page may simply result in it being
inherited from another. This is unusual and counterintuitive
behavior and can result in subtle bugs related to specific
documents. As such, if you want to clear this property you should
assign it the value of the MediaBox.