The problem may be due to the fact that the buttons change the scale factor to a value that may not be included in the combo box. You can verify that by adding a TextBox bound to the scale factor of the editor and see the value of the property.
When it comes to the references you have mentioned, RadPdfViewer version 2012.1.326 requires the following assemblies:
If this does not help, we will appreciate if you could send us a demo application attached to a support ticket to better understand the problem.
the Telerik team
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