How can I create notes inside an existing PDF document?

Q:
How can I create notes inside an existing PDF document?
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A:

You can use PDFNet SDK to create notes inside an existing PDF document
(from a webpage or a desktop application). You may want to take a look
at Annotation sample project (http://www.pdftron.com/net/
samplecode.html#Annotation), which illustrates how to create new
annotations or to process annotations in existing PDF documents.

Based on your requirements, it seems that you would need to create a
'text' annotation. In pdftron.PDF.Annot class there is a utility
method to create this type of annotation (try
pdftron.PDF.Annot.CreateText (pdfdoc, new Rect(10, 10, 300, 110), "My
note...")). PDFNet SDK also offers a simple to use API to create
annotations and properties that are not directly supported in high-
level interfaces.