//Here is my code and I face issue of Object reference null in below given statement
//pdfDocument.EmbeddedFiles.Add(fileSpecification);
string dataDir = Server.MapPath("~/Doc");
Document pdfDocument = new Document(dataDir + "//test.pdf");
// Setup new file to be added as attachment
FileSpecification fileSpecification = new FileSpecification(dataDir + "//Aetna.txt", "Sample text file");
// Specify Encoding proparty setting it to FileEncoding.None
fileSpecification.Encoding = FileEncoding.None;
// Add attachment to document's attachment collection
pdfDocument.EmbeddedFiles.Add(fileSpecification);
dataDir = dataDir + "DisableFilesCompression_out.pdf";
// Save new output
pdfDocument.Save(dataDir);