private void btnGetAIThumb_Click(object sender, EventArgs e)
{
    Illustrator.Application app = new Illustrator.Application();
    Illustrator.Document doc = app.Open(@"F:/AI_Prog/2009Calendar.ai", Illustrator.AiDocumentColorSpace.aiDocumentRGBColor, null);
    doc.Export(@"F:/AI_Prog/2009Calendar.png",Illustrator.AiExportType.aiPNG24, null);
    doc.Close(Illustrator.AiSaveOptions.aiDoNotSaveChanges);
    doc = null; //
}

string pdf_filename = @"C:test.ai";
//string pdf_filename = @"C:test.pdf";
string png_filename = "converted.png";
List<string> errors = cs_pdf_to_image.Pdf2Image.Convert(pdf_filename, png_filename);