Hi Alex,
You can design margins and padding's like below;
Document.DefaultPageSetup.Orientation = MigraDoc.DocumentObjectModel.Orientation.Portrait;
Document.DefaultPageSetup.TopMargin = "10mm";
Document.DefaultPageSetup.LeftMargin = "5mm";
Document.DefaultPageSetup.RightMargin = "5mm";
Try something like above to adjust Top, Left and right margins.