PDF coordinates are measured upwards from the bottom of the
document. For some types of layout, it can be useful to measure
coordinates downwards from the top of the document.
By setting this property to true, coordinates are assumed to
start at the top rather than the bottom of the document.
More precisely, the origin is assumed to be at the top-left of
the current MediaBox.
There are a variety of methods you
can use to change coordinate systems.
Note that transforms operate on the underlying PDF coordinate
space rather than any abstraction specified by the Units and TopDown
properties. If you are using transforms, you will find it easiest
to work in the native PDF coordinate space.