I have ever tried to extract text from PDF files with the help of the following code:
     using YiiGo.Imaging.Basic;
     using YiiGo.Imaging.Basic.Core;
     using YiiGo.Imaging.Basic.Codec;
     using YiiGo.Imaging.PDF;
     YiiGoImaging PDF = new YiiGoImaging();
     public void PdfProcessorExtractTextPage();
     {
     PDFInputFile = (@"C:/1.pdf");
     PDFPageNumberStart = "0";
     PDFPageNumberStop = "4";
     PDFOutputFile = OutputFormat.txt;
     PDFOutputFile = (@"C:/extract.txt");
     };
     PDF. PdfProcessorExtractText (@"C:/1.pdf", "0","4",  @"C:/extract.txt");
You can check its tutorial page here: 
yiigo.com/guides/csharp/how-to-extr ...
I hope it helps. Good luck.
Best regards,
Arron