To fix the issue put the following script tag:

 <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/pdf.js/2.2.2/pdf.worker.js"></script>

just below the one:

  <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/pdf.js/2.2.2/pdf.js"></script>

Hello Daniel,

Thank you for the reported issue!

As a possible workaround until this bug is fixed you can use the Kendo UI PDF viewer directly as the faced problem seems to be not reproducible there:
https://demos.telerik.com/aspnet-ajax/pdfviewer/overview/defaultcs.aspx

In case you have other controls form the UI for ASP.NET AJAX suite on the page, you will need to apply the settings from the following help article in order to use them along with the Kendo UI widgets smoothly:
https://docs.telerik.com/devtools/aspnet-ajax/controls/htmlchart/how-to/radhtmlchart-integration-with-kendoui-widgets

Regards,
Vessy
Progress Telerik