Kiosk Browser uses WebView, a built-in system web engine. It works with HTML only and is not supposed to display PDF in an embedded window. Currently it treats PDF as external files and opens the related application.
To avoid opening PDF in Chrome, install a dedicated application to view the PDF files only. Or avoid PDF in your web application. Note that Kiosk Browser is designed as an engine for standalone kiosks which aren't supposed to display PDF to their users.