The document Units and TopDown properties
allow you to use units of measurement like mm or cm. They also
allow you to invert the coordinate system so that distances are
measured down from the top rather than up from the bottom.
These properties enable an abstraction designed to make layout
easier. However the underlying units of measurement and page layout
remain the same.
When you call a page layout operation like AddText or FrameRect your
abstracted coordinates are translated into PDF coordinates before
the object is inserted into the document.
For most layout tasks you can ignore the translation. However if
you are using low-level access to the PDF structure or if you are
then you need to be aware of it.
Suppose you call FrameRect and then
extract the raw content stream from the returned object. You must
be aware that the base coordinates included in the stream are
measured in the PDF coordinate space and not in your current
Document Transforms operate on the native PDF coordinate space.
So when you specify a rotation around the origin - the origin
specified is at the bottom left of the document - not the top.