Program.cs

using System.IO;
using TallComponents.PDF;
using TallComponents.PDF.Forms.Fields;
using TallComponents.PDF.JavaScript;

Console.WriteLine(System.IO.Directory.GetCurrentDirectory());

using (FileStream inFile = new FileStream(@"./nIVD_eSTAR_40.pdf", FileMode.Open, FileAccess.Read))
{
    Document document = new Document(inFile);
    document.Fields.Changed += Fields_Changed;

    var fields = document.Fields;
    Console.WriteLine("Before we click the button: " + fields.Count);
   

    RadioButtonField radioField = document.Fields["root[0].ApplicationType[0].USA[0].ATRadioButton110[0]"] as RadioButtonField;
    radioField.Value = "1";

    radioField = document.Fields["root[0].ApplicationType[0].USA[0].ATRadioButton115[0]"] as RadioButtonField;
    radioField.Value = "1";

    radioField = document.Fields["root[0].ApplicationType[0].ApplicationSubType[0].ATRadioButton130[0]"] as RadioButtonField;
    radioField.Value = "1";

    DropDownListField dropDown = document.Fields["root[0].AdministrativeInformation[0].ApplicantInformation[0].ADDropDownList120[0]"] as DropDownListField;
    dropDown.Value = "1";

    TextField textField = document.Fields["root[0].AdministrativeInformation[0].ApplicantInformation[0].ADTextField130[0]"] as TextField;
    textField.Value = "Bruce";

    textField = document.Fields["root[0].AdministrativeInformation[0].ApplicantInformation[0].ADTextField140[0]"] as TextField;
    textField.Value = "Wayne";

    Console.WriteLine("After we click the button: " + document.Fields.Count);

    FileStream stream = File.Open(@"./sample.pdf", FileMode.Open);
    EmbeddedFile ef = new EmbeddedFile(stream, "Sample PDF");
    document.EmbeddedFiles.Add(ef);

    Console.WriteLine("Before:");
    for (int i = 0; i < fields.Count; i++)
    {
        Console.WriteLine("\t" + document.Fields[i].FullName);
    }

    Console.WriteLine("After:");
    for (int i = 0; i < document.Fields.Count; i++)
    {
        Console.WriteLine("\t" + document.Fields[i].FullName);
    }

    using (FileStream outFile = new FileStream(@"./nIVD_eSTAR_40-filled.pdf", FileMode.Create, FileAccess.Write))
    {
        document.Write(outFile);
    }

}

Console.WriteLine("All done!");

static void Fields_Changed(FieldCollection sender, FieldsChangedEventArgs e)
{
    Console.WriteLine(e.Added.Length);
}