使用iTextSharp将水印添加到PDF不适用于图像PDF C#

using iTextSharp.text;
using iTextSharp.text.pdf;

string inputFilePath = "path/to/input.pdf";
string outputFilePath = "path/to/output.pdf";

PdfReader reader = new PdfReader(inputFilePath);
PdfStamper stamper = new PdfStamper(reader, new FileStream(outputFilePath, FileMode.Create));

BaseFont baseFont = BaseFont.CreateFont(BaseFont.HELVETICA, BaseFont.WINANSI, BaseFont.EMBEDDED);
Font font = new Font(baseFont, 48, Font.BOLD, BaseColor.LIGHT_GRAY);

int pageCount = reader.NumberOfPages;

for (int i = 1; i <= pageCount; i++)
{
    PdfContentByte content = stamper.GetUnderContent(i);
    ColumnText.ShowTextAligned(content, Element.ALIGN_CENTER, new Phrase("水印文字", font), 297, 421, 45);
}

stamper.Close();
reader.Close();