When viewing a pdf file in krusader (by hitting F3), text is shown. When switching to the generic mode (Ctrl+Shift+G), the message "No file selected or impossible to view the selected file" is shown.
Let me add that Okular is installed and working, what used to be the PDF Viewer used by the generic viewer to show PDFs.
I get exactly the same behaviour in debian sid updated 2017-03-11. Double-clicking the pdf file Okular opens fine outside of viewer. Krusader v2.5.0, KDE Frameworks 5.28.0, Qt 5.7.1 (built against 5.7.1).
Porting Okular to KF5 needed some time but is done now. With KDE Applications 16.12 it should work again.