pdf viewer is not working on nano browser and simple browser sample apps

<Issue>
According to https://doc.qt.io/qt-6/qtwebengine-features.html#pdf-file-viewing, both nano brower(https://doc.qt.io/qt-6/qtwebengine-webenginequick-quicknanobrowser-example.html) and simple brower(https://doc.qt.io/qt-6/qtwebengine-webenginewidgets-simplebrowser-example.html) sample apps should be able to display pdf files. However, they don't; they just show the gray screen on the browser.

<Reproduction Steps>

1. Open either nano browser or simple browser sample and configure & run it with Desktop Qt 6.8.0 MSVC2022 64bit Kit.
2. Drag and drop the attached pdf files to the browsers.
   1. 28.5 — Stream states and input validation – Learn C++.pdf
   2. 28.4 — Stream classes for strings – Learn C++.pdf
   3. 28.3 — Output with ostream and ios – Learn C++.pdf

<Expected Result>
The pdf file shows in the brower.

<Actual Result - nano browser>

<Actual Result - simple browser>