@ravina MS Word documents are flow by their nature, so there is no Page concept. You can use a separate section for the first page to specify different margins. For example see the following code:
Document doc = new Document();
DocumentBuilder builder = new DocumentBuilder(doc);
builder.PageSetup.BottomMargin = 10;
builder.PageSetup.TopMargin = 10;
builder.PageSetup.LeftMargin = 20;
builder.PageSetup.RightMargin = 10;
builder.PageSetup.Orientation = Orientation.Portrait;
builder.Write("This is the first page");
// Insert section break.
builder.InsertBreak(BreakType.SectionBreakNewPage);
builder.PageSetup.BottomMargin = 50;
builder.PageSetup.TopMargin = 50;
builder.PageSetup.LeftMargin = 50;
builder.PageSetup.RightMargin = 50;
builder.PageSetup.Orientation = Orientation.Landscape;
builder.Write("This is the second page");
doc.Save(@"C:\Temp\out.docx");