I am not sure how you are creating your project. A reference to the Telerik.Windows.Controls.FixedDocumentViewers.dll could be automatically added if you drag the RadPdfViewer control from the toolbox in Visual Studio and drop it in the designer or create a Telerik project and choose the referenced assemblies through the provided list in the Project Configuration Wizard. If you choose the Telerik.Windows.Controls.FixedDocumentViewersUI.dll, all the required assemblies will be referenced automatically.
In addition, you may find helpful to check the Installation and Deployment section
in our documentation, where is described how to install, setup and use the controls. More information about the RadPdfViewer control is available here
Hope this is helpful.
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