Objects added to a page are stored as individual layers. Calling
this method combines all the layers on the current page and then
re-compresses the layer data.
For pages that contain only a few layers the reduction in size
will be minimal. However for pages which contain complex tables
with many items, flattening can reduce the size of the output PDF
by a factor of five or more.
Note that flattening will delete all the items currently on the
page and replace them with a new compressed item. This means that
Object IDs previously obtained from calls such as AddText or FrameRect
will no longer be valid.