try this code I think it is more optimal.
HeaderRow is used to repeat the header of the table for each new page automatically
BaseFont bfTimes = BaseFont.CreateFont(BaseFont.TIMES_ROMAN, BaseFont.CP1252, false);
iTextSharp.text.Font times = new iTextSharp.text.Font(bfTimes, 6, iTextSharp.text.Font.NORMAL, iTextSharp.text.BaseColor.BLACK);
PdfPTable table = new PdfPTable(10) { HorizontalAlignment = Element.ALIGN_CENTER, WidthPercentage = 100, HeaderRows = 2 };
table.SetWidths(new float[] { 2f, 6f, 6f, 3f, 5f, 8f, 5f, 5f, 5f, 5f });
table.AddCell(new PdfPCell(new Phrase("SER.\nNO.", times)) { Rowspan = 2, GrayFill = 0.95f });
table.AddCell(new PdfPCell(new Phrase("TYPE OF SHIPPING", times)) { GrayFill = 0.95f });
table.AddCell(new PdfPCell(new Phrase("ORDER NO.", times)) { GrayFill = 0.95f });
table.AddCell(new PdfPCell(new Phrase("QTY.", times)) { GrayFill = 0.95f });
table.AddCell(new PdfPCell(new Phrase("DISCHARGE PPORT", times)) { GrayFill = 0.95f });
table.AddCell(new PdfPCell(new Phrase("DESCRIPTION OF GOODS", times)) { Rowspan = 2, GrayFill = 0.95f });
table.AddCell(new PdfPCell(new Phrase("LINE DOC. RECL DATE", times)) { GrayFill = 0.95f });
table.AddCell(new PdfPCell(new Phrase("CLEARANCE DATE", times)) { Rowspan = 2, GrayFill = 0.95f });
table.AddCell(new PdfPCell(new Phrase("CUSTOM PERMIT NO.", times)) { Rowspan = 2, GrayFill = 0.95f });
table.AddCell(new PdfPCell(new Phrase("DISPATCH DATE", times)) { Rowspan = 2, GrayFill = 0.95f });
table.AddCell(new PdfPCell(new Phrase("AWB/BL NO.", times)) { GrayFill = 0.95f });
table.AddCell(new PdfPCell(new Phrase("COMPLEX NAME", times)) { GrayFill = 0.95f });
table.AddCell(new PdfPCell(new Phrase("G. W. Kgs.", times)) { GrayFill = 0.95f });
table.AddCell(new PdfPCell(new Phrase("DESTINATION", times)) { GrayFill = 0.95f });
table.AddCell(new PdfPCell(new Phrase("OWNER DOC. RECL DATE", times)) { GrayFill = 0.95f });