To add custom data to an existing document.

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

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

// Create a custom inline dictionary within Info dictionary
customDict := info.PutDict("My Direct Dict")
customDict.PutNumber("My Number", 100)     // Add some key/value pairs

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.