PdfFileAttachment.Data Property

using DevExpress.Pdf;
using System;
using System.IO;

namespace AttachFile {
    class Program {
        static void Main(string[] args) {

            using (PdfDocumentProcessor processor = new PdfDocumentProcessor()) {
                // Load a document.
                processor.LoadDocument("..\\..\\Document.pdf");

                // Attach a file to the PDF document. 
                processor.AttachFile(new PdfFileAttachment() {
                    CreationDate = DateTime.Now,
                    Description = "This is my attach file.",
                    FileName = "MyAttach.txt",
                    Data = File.ReadAllBytes("..\\..\\FileToAttach.txt")
                });

                // The attached document.
                processor.SaveDocument("..\\..\\Result.pdf");
            }
        }
    }
}

Imports DevExpress.Pdf
Imports System
Imports System.IO

Namespace AttachFile
    Friend Class Program
        Shared Sub Main(ByVal args() As String)

            Using processor As New PdfDocumentProcessor()
                ' Load a document.
                processor.LoadDocument("..\..\Document.pdf")

                ' Attach a file to the PDF document. 
                processor.AttachFile(New PdfFileAttachment() With { _
                    .CreationDate = Date.Now, _
                    .Description = "This is my attach file.", _
                    .FileName = "MyAttach.txt", _
                    .Data = File.ReadAllBytes("..\..\FileToAttach.txt") _
                })

                ' The attached document.
                processor.SaveDocument("..\..\Result.pdf")
            End Using
        End Sub
    End Class
End Namespace