Adds a new page to the document. Returns an instance of the PdfPage
object representing the newly created page.
Width, Height - optionally specify
the width and height (in default user units) of the page.
By default, the page size is 612 x 792 which corresponds
to 8.5" x 11".
InsertBefore is an optional 1-based index of a page
before which the new page will be inserted.