程序代码：

using System.Text;
using System.Xml;
using Spire.Pdf;
namespace _test
{
    class Program
    {
        static void Main(string[] args)
        {
            PdfDocument doc = new PdfDocument();

            //加载PDF文档
            doc.LoadFromFile("D:\\test.pdf");

            StringBuilder content = new StringBuilder();

            //提取PDF所有页的文本
            foreach (PdfPageBase page in doc.Pages)
            {
                content.Append(page.ExtractText());
            }

            String fileName = "D:\\获取文本.txt";
            File.WriteAllText(fileName, content.ToString());
            }
    }
}