实现思路：

步骤一、根据PDF地址下载pdf文件保存为临时文件，获得pdf文件的byte[]数组

/// 从指定的 URL 下载 PDF 文件public  byte[] DownloadPdf(string url){try{using (WebClient client = new WebClient()){return client.DownloadData(url);}}catch (Exception ex){Console.WriteLine($"下载PDF异常: {ex.Message}");return null;}}

步骤二、初始化pdfDocument类，并加载PDF文档，遍历pdf文件，一页生成一个jpg图片，并保存图片

 string url = infoEntity.ReportUrl; //pdf地址byte[] pdfBytes = DownloadPdf(url);string tempPdfPath = Path.GetTempFileName();File.WriteAllBytes(tempPdfPath, pdfBytes);// 初始化一个PdfDocument类实例，并加载PDF文档
PdfDocument doc = new PdfDocument();
doc.LoadFromFile(tempPdfPath);
// 遍历PDF每一页for (int i = 0; i < doc.Pages.Count; i++){Report_Pic report_Pic = new Report_Pic();// 将PDF页转换成Bitmap图形System.Drawing.Image bmp = doc.SaveAsImage(i);string imgPath = Application.StartupPath;// 将Bitmap图形保存为jpg格式的图片 string pngPath = $"{imgPath}\\img\\{infoEntity.ReportFileName.Replace(".pdf","")}_page_{i}.jpg";if (File.Exists(pngPath)){try{File.Delete(pngPath);}catch (Exception ex){Log.error($"删除图片时出错: {ex.Message}");}}bmp.Save(pngPath, System.Drawing.Imaging.ImageFormat.Jpeg);Log.info("PDF地址："+tempPdfPath+"    图片地址：" + pngPath);string imageBase64Str= ConvertImageToBase64(bmp); //图片内容[Base64编码]
Log.info("图片Base64编码："+imageBase64Str);//删除图片
File.Delete(pngPath);
}
// 删除临时文件
File.Delete(tempPdfPath);

步骤三、将每张图片转换成Base64编码

 // 将图片转换为 Base64 编码static string ConvertImageToBase64(Image image){using (MemoryStream ms = new MemoryStream()){image.Save(ms, ImageFormat.Jpeg);byte[] imageBytes = ms.ToArray();return Convert.ToBase64String(imageBytes);}}

备注：

目标框架：.Net Framework 4

引用库

using System.Net;
using System.Drawing;  //版本  4.0.0.0
using System.Drawing.Imaging;
using Spire.Pdf;   //版本8.6.1.0