To reproduce the bug you can use the pdfviewer example from Qt 5.15.1 package. Its location is qtrootfolder\Examples\Qt-5.15.1\pdf\pdfviewer. I did not modify the example's code in any way.

When I try to open some pdf file from my HD I always get an error "File not found".

I debuged the issue and I've found that the path which is passed to QFile constructor had an extra "/" at the beginning.  In my case the path was "/D:/Cpp/pdf/java.pdf".  Look at the screenshot attached to the bug report for more details. The issue is rather annoying, actually it blocks the usage of local files in qml.