test.cs

// convert PDF to byte array
byte[] baAttachment = System.IO.File.ReadAllBytes("forms.pdf");

// create a new embedded file
TXTextControl.EmbeddedFile efAttachment = 
    new TXTextControl.EmbeddedFile("myInvoice.pdf", baAttachment, null)
{
    Description = "My embedded file."
};

// add the embedded file to TextControl
textControl1.DocumentSettings.EmbeddedFiles = 
    new TXTextControl.EmbeddedFile[] { efAttachment };

// save the document
textControl1.Save("mypdf.pdf", TXTextControl.StreamType.AdobePDF);