It is not possible to modify the content of the PDF document in RadPdfViewer, as it is a viewer and has no implementation of content-changing commands/operations. That is why if you want to save the PDF file that is opened in the editor, you just need to write the stream that has been used to create the document in the first place. You can refer to this forum post for a demo of how custom Open and Save commands can be implemented, so as to achieve this behavior.
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