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 = SDFDoc(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
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.