PeachPDF
PeachPDF.Layout.IDocumentBuilder
IDocumentBuilder.UnderlayFile(Action<IFileLayerDescriptor>) Method
Stamps pages of an existing PDF file under the document’s pages, before their own content is drawn - a letterhead or a form background that the page’s content then sits on. See OverlayFile(Action<IFileLayerDescriptor>) for what is carried over.
void UnderlayFile(System.Action<PeachPDF.Layout.IFileLayerDescriptor> handler);