string myPath = AppDomain.CurrentDomain.BaseDirectory;
path += "MyDocuments\";
Process.Start(System.IO.Path.Combine(myPath, "MyPDF.pdf"));

String pathToAcroRd32 = Environment.GetEnvironmentVariable("ProgramFiles") + ((Environment.Is64BitOperatingSystem) ? @" (x86)" : @"") + "Adobe\Reader 11.0\Reader\AcroRd32.exe";
ProcessStartInfo adobeInfo = new ProcessStartInfo(pathToAcroRd32, System.IO.Path.Combine(myPath, "MyPDF.pdf"));
Process.Start(adobeInfo);