I am being using RadPDFViewer control in one of our Silverlight screen for loading PDF reports. While testing we observed that for large pdf files of size approx. 15mb or above (Total pages 10k+), the find dialog fails to search the specified text search after 3-4 attempts. In our case for the Report of total pages 18000, the search option navigated fine for the first 3 searched locations and then failed to proceed further. I even tried to test the same behavior on your demo site
(http://demos.telerik.com/silverlight/#PdfViewer/FirstLook) with the same report and observed that there too the find dialog failed to perform search after 3 navigations.
Could you please take a look into the issue and suggest further. We have been using the RadControls for Silverlight Q3 2013 version.