From the snippet you shared, it seems like you are using the API of RadRichTextBox. If so, the control currently doesn't support protecting PDF documents with a password and we have logged a feature request to add this functionality on our public portal. You can vote for the implementation as well as track the status of the task using this feedback item
class is part of the PdfProcessing
library, from the Telerik Document Processing
package. You could use it to import the generated from RadRichTextBox PDF document and apply it a password. Please, note that this library is released with Q2 2014
and you will need to get a newer version of the controls to use it.
Hope this helps.
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