PdfPTable table = new PdfPTable(3);
PdfPCell cell = new PdfPCell(new Phrase("Header spanning 3 columns"));
cell.Colspan = 3;
cell.HorizontalAlignment = 1; //0=Left, 1=Centre, 2=Right
table.AddCell(cell);
table.AddCell("Col 1 Row 1");
table.AddCell("Col 2 Row 1");
table.AddCell("Col 3 Row 1");
table.AddCell("Col 1 Row 2");
table.AddCell("Col 2 Row 2");
table.AddCell("Col 3 Row 2");
doc.Add(table);