To add custom data to an existing document.

// In case you have PDFDoc you can always access
// SDF/Cos document using PDFDoc.GetSDFDoc() method
SDFDoc doc = new SDFDoc(filename);
Obj trailer = doc.GetTrailer();	// Get the trailer

// Create an Info dictionary
Obj info = trailer.PutDict("Info");
info.PutString("Producer", "Apryse PDFNet");

// Create a custom inline dictionary within Info dictionary
Obj custom_dict = info.PutDict("My Direct Dict");
custom_dict.PutNumber("My Number", 100);

Cos/SDF low-level API to edit PDF files
Full code sample which illustrates how to use basic Cos/SDF API to edit an existing document.