PdfDocument doc = new PdfDocument();
doc.LoadFromFile(input);
PdfPageBase pdfPageBase = doc.Pages[0];
PdfTextFinder finder = new PdfTextFinder(pdfPageBase);
finder.Options.Strategy = PdfTextStrategy.Simple;

PdfDocument doc = new PdfDocument();
PdfPageBase page = doc.Pages.Add();
string text = "Please add the code picture BehindText=true to set the picture behind text";
PdfSolidBrush solidBrush = new PdfSolidBrush (new PdfRGBColor(Color.Black));
PdfStringFormat format = new PdfStringFormat():
format.HorizontalScalingFactor = 80;
PdfFont font = new PdfFont(PdfFontFamily.TimesRoman, 14f, PdfFontStyle.Regular);
PdfGraphicsState state = page.Canvas.Save();
page.Canvas.DrawString(text, font, solidBrush, 0, 0, format);
page.Canvas.Restore(state):
doc.SaveToFile(outputFile);
doc.Close();

PdfDocument doc = new PdfDocument();
doc.LoadFromFile(inputFile);
doc.ConvertOptions.ApplyPermissionsOptions(true);
StringBuilder sb = new StringBuilder();
    foreach (FileFormat type in Enum.GetValues(typeof(FileFormat)))
        {
            try
                {
                    if (type.ToString().Equals("PDF"))
                    {
                        doc.SaveToFile(outputFile_P, type);
                    }
                    else
                    {
                        doc.SaveToFile(outputFile, type);
                    }
                }
                catch (Exception ex)
                {
                    sb.AppendLine("save to： "+ type +"  :"+ ex.Message);
                }
            }
File.AppendAllText(outputFile,sb.ToString());
doc.Dispose();