Short answer: If you want to view a PDF in a hybrid app, you might simply use the “download” activity in a microflow and let the device’s operating system open your file.
Long answer:
I think this is happening because of a limitation in the current widget. The widget leverages the PDF.js library, and it doesn’t like when your HTML page and the PDF document are not hosted at the same URL.
In the case of a hybrid app, your HTML page lives on the device itself, while the document lives on the Mendix server. Because there’s a mismatch, you get the error here.
I found this related discussion about the PDF.js module that the document view widget uses. It’s also an open issue on the widget itself.