安装这两个库，第二个库一定要安装否则有些pdf文件读取会出现异常

读取

using iText.Kernel.Pdf;
using iText.Kernel.Pdf.Canvas.Parser;
using iText.Kernel.Pdf.Canvas.Parser.Listener;

namespace TestReadPdf
{
    public static class PdfHelper
    {
        public static IEnumerable<string> ExtractText(string filename)
        {
            using (var r = new PdfReader(filename))
            using (var doc = new PdfDocument(r))
            {
                for (int i = 1; i < doc.GetNumberOfPages(); i++)
                {
                    ITextExtractionStrategy strategy = new LocationTextExtractionStrategy();
                    string text = PdfTextExtractor.GetTextFromPage(doc.GetPage(i), strategy);
                    yield return text;
                }
            }
        }
    }
}