We do not plan to implement loading of PDF documents in RadRichTextBox, as the latter is an editor for flow documents, which are quite different from fixed ones (which PDF represents).
For the next official version we are planning to improve our compatibility with PDF and expose an API for creation of custom FixedDocuments. This API can be used for merging documents, but it would not be possible to save the result as a file - this will be eventually implemented in future releases.
Filling PDF templates is quite a complex task and for now it is out of the control's planned scope.
Don't hesitate to contact us if you have other questions about RadPdfViewer.
the Telerik team
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