page = doc:AddPage()
doc:AddPage() creates a new page and adds it after the last page of a document.
doc:AddPage() returns the handle of created page object on success.
Otherwise, it returns an error code and the error handler is called.
#HPDF_INVALID_DOCUMENT - An invalid document handle was set.
#HPDF_FAILED_TO_ALLOC_MEM - Memory allocation failed.