itextsharp獲取PDF頁碼
PdfReader reader;
reader = new PdfReader(fileList[i]);
int iPageNum = reader.NumberOfPages;

提取PDF中的文本和圖片(參考)：https://www.e-iceblue.cn/spirepdf/extract-text-and-image-from-pdf.html

Spire獲取PDF文件圖片：

public Image GetPDFImg(string filePath, int imgSequence)
{
    Image image = null;
    try
    {
        //加載PDF文檔
        Spire.Pdf.PdfDocument doc = new Spire.Pdf.PdfDocument();
        doc.LoadFromFile(filePath);

        List<Image> ListImage = new List<Image>();

        // 實例化一個Spire.Pdf.PdfPageBase對象,Pages從0開始
        Spire.Pdf.PdfPageBase page = doc.Pages[imgSequence];

        // 獲取PDF第一頁里面的所有圖片
        Image[] images = page.ExtractImages();
        if (images != null && images.Length > 0)
        {
            ListImage.AddRange(images);
        }

        if (ListImage.Count > 0)
        {
            image = ListImage[0];
            //image.Save(@"D:\temp\" + GetDateTimeStr() + ".png", System.Drawing.Imaging.ImageFormat.Png);//保存獲取的圖片
        }
        doc.Dispose();
    }
    catch (Exception e)
    {
        MessageBox.Show(e.ToString(), "錯誤提示");
    }

    return image;
}

itextsharp獲取PDF文件圖片：

public void ExtractImage(string pdfFile)
{

    PdfReader pdfReader = new PdfReader(pdfFile);

    for (int pageNumber = 1; pageNumber <= pdfReader.NumberOfPages; pageNumber++)
    {

        PdfReader pdf = new PdfReader(pdfFile);

        PdfDictionary pg = pdf.GetPageN(pageNumber);

        PdfDictionary res = (PdfDictionary)PdfReader.GetPdfObject(pg.Get(PdfName.RESOURCES));

        PdfDictionary xobj = (PdfDictionary)PdfReader.GetPdfObject(res.Get(PdfName.XOBJECT));

        try
        {

            foreach (PdfName name in xobj.Keys)
            {

                PdfObject obj = xobj.Get(name);

                if (obj.IsIndirect())
                {

                    PdfDictionary tg = (PdfDictionary)PdfReader.GetPdfObject(obj);

                    string width = tg.Get(PdfName.WIDTH).ToString();

                    string height = tg.Get(PdfName.HEIGHT).ToString();

                    ImageRenderInfo imgRI = ImageRenderInfo.CreateForXObject(new Matrix(), (PRIndirectReference)obj, tg);


                    //圖片保存
                    PdfImageObject image = imgRI.GetImage();
                    Image dotnetImg = image.GetDrawingImage();
                    dotnetImg.Save(@"E:\\000.png");
                }
            }
        }
        catch (Exception e)
        {
            MessageBox.Show(e.ToString(), "錯誤提示");
        }

        break;
    }
}

合并PDF文件

private void mergePDFFiles(string[] fileList,string outMergeFile)
{
    //try
    //{
        List<PdfReader> readerList = new List<PdfReader>();//記錄合并PDF集合
        PdfReader reader;
        Document document = new Document();
        PdfWriter writer = PdfWriter.GetInstance(document, new FileStream(outMergeFile, FileMode.Create));
        document.Open();
        PdfContentByte cb = writer.DirectContent;
        PdfImportedPage newPage;
        PdfDictionary page;
        PdfNumber rotate;

        for (int i = 0; i < fileList.Length; i++)
        {
            if (fileList[i] != null)
            {
                try
                {
                    reader = new PdfReader(fileList[i]);
                    int iPageNum = reader.NumberOfPages;
                    for (int j = 1; j <= iPageNum; j++)
                    {
                        newPage = writer.GetImportedPage(reader, j);
                        page = reader.GetPageN(j);
                        rotate = page.GetAsNumber(PdfName.ROTATE);

                        iTextSharp.text.Rectangle r = reader.GetPageSizeWithRotation(j);

                        document.SetPageSize(r);
                        document.NewPage();

                        //解決PDF合并時，頁面翻轉(zhuǎn)問題
                        if (rotate == null)
                        {
                            cb.AddTemplate(newPage, 1f, 0, 0, 1f, 0, 0);
                        }
                        else
                        {
                            switch (rotate.IntValue)
                            {
                                case 0:
                                    cb.AddTemplate(newPage, 1f, 0, 0, 1f, 0, 0);
                                    break;
                                case 90:
                                    cb.AddTemplate(newPage, 0, -1f, 1f, 0, 0, r.Height);
                                    break;
                                case 180:
                                    cb.AddTemplate(newPage, -1f, 0, 0, -1f, r.Width, r.Height);
                                    break;
                                case 270:
                                    cb.AddTemplate(newPage, 0, 1f, -1f, 0, r.Height, 0);
                                    break;
                                //default:
                                //    throw new InvalidOperationException(string.Format("Unexpected page rotation: [{0}].", pageRotation));
                            }
                        }
                    }
                    readerList.Add(reader);
                }
                catch
                {
                }
            }
        }
        document.Close();
        for (int i = 0; i < readerList.Count; i++)
        {
            readerList[i].Dispose();
        }
    //}
    //catch (Exception)
    //{
    //    MessageBox.Show("合并失敗彤路，請聯(lián)系管理員俭正！", "提示");
    //}
}

獲取PDF文字信息

using iTextSharp.text.pdf;
using iTextSharp.text.pdf.parser;
using System.Diagnostics;
List<string> lst1 = new List<string>();
lst1.Add(@"C:\Users\253\Desktop\20211118\aaa.pdf");

for(int i=0;i<lst1.Count;i++)
{
    PdfReader pdfReader = new PdfReader(lst1[i]);

    iTextSharp.text.Rectangle rect = new iTextSharp.text.Rectangle(400, 400, 800, 800);//獲取指定區(qū)域文字信息

    RenderFilter[] filter = { new RegionTextRenderFilter(rect) };

    ITextExtractionStrategy strategy = new FilteredTextRenderListener(new LocationTextExtractionStrategy(), filter);

    string atext = PdfTextExtractor.GetTextFromPage(pdfReader, 1, strategy);
    atext = atext.Replace(" ", "").Replace("發(fā)文編號：\n", "");
    atext = atext.Substring(0, atext.IndexOf("\n"));
    pdfReader.Close();
}

//PdfReader pdfReader = new PdfReader(@"C:\Users\253\Desktop\20211118 \aaa.pdf");
//int numberOfPages = pdfReader.NumberOfPages;
//StringBuilder text = new StringBuilder();
//for (int i = 1; i <= numberOfPages; ++i)
//{
//    text.Append(iTextSharp.text.pdf.parser.PdfTextExtractor.GetTextFromPage(pdfReader, i));//獲取指定頁全部文字信息
//    break;
//}
//pdfReader.Close();