I can't test with PowerPDF that I don't have but I faced the same issue with the Adobe's Acrobat SDK itself (I guess this PowerPDF uses Acrobat SDK undercovers).

Anyway, in the Acrobat case, the C# dynamic "pseudo-type" doesn't work because the underlying object implementing COM Automation IDispatch always fails (and actually causes a crash in Acrobat process...) when we call GetTypeInfo, which is what dynamic does in its implementation.

The solution is to use what we did before dynamic existed: use direct Reflection techniques like this (you may have to adapt it slightly to PowerPDF):

  static void Main()
  {
      Type GetPDFType = Type.GetTypeFromProgID("AcroExch.PDDoc");
      dynamic dvOpenDoc = Activator.CreateInstance(GetPDFType);
      dvOpenDoc.Open(@"C:\TestFiles\SampleFormSource.pdf");

      object jso = dvOpenDoc.GetJSObject(); // note I use object type here, not dynamic!

      // these reflection calls are equivalent to jso.app.alert("Show this alert");

      // get the "app" property
      var app = jso.GetType().InvokeMember("app", BindingFlags.Instance | BindingFlags.GetProperty | BindingFlags.Public, null, jso, null);

      // call the "alert" method (has 1 argument)
      app.GetType().InvokeMember("alert", BindingFlags.Instance | BindingFlags.InvokeMethod | BindingFlags.Public, null, app, new[] { "Show this alert" });
      return;
  }