how to search .pdf file from the particular folder ad opening that file

You should split your requirement in two parts:
- getting a list of PDF files in a specific folder
- opening a specific file with default system handler

For the first one, you have to use object in System.IO namespace.

using System.IO;

/// <summary>
/// Retrieves a list of files with a specific extension in a specific folder.
/// </summary>
/// <param name="path">Directory path in which to search for files.</param>
/// <param name="extension">File extension to search for.</param>
/// <returns>An IEnumerable list of files with <paramref name="extension" /> in
/// <paramref name="path" />.</returns>
public static IEnumerable<FileInfo> GetFiles(string path, string extension) {
   DirectoryInfo di = new DirectoryInfo(path);
   if (di.Exists) {
      string search = string.Format("*.{0}", extension);
      FileInfo[] files = di.GetFiles(search);
      foreach (FileInfo fi in files) {
         yield return fi;
      }
   }
}

So, to get all PDF files in "C:\Test" folder, you would write:

IEnumerable<FileInfo> pdfFiles = GetFiles(@"C:\Test", "pdf");

Then you want to open the file(s) obtained; this way:

foreach (FileInfo fi in pdfFiles) {
   ProcessStartInfo psi = new ProcessStartInfo(fi.FileName);
   psi.UseShellExecute = true;
   Process.Start(psi);
}

Hope this helps. Good luck!