In short: the standalone viewer does not open single focused PDF file if there's at least one image file in the same folder.
Here are a few examples of what happens on my system:
A) In a folder that contains just a single PDF file or a PDF and some random non-image files, if I focus the PDF and invoke the standalone viewer (via Slideshow -> Show Pictures), the PDF will be displayed just fine.
B) In a folder that contains one or more PDF files and at least one image file, if I focus one of the PDF files and attempt to invoke the standalone viewer, it opens and displays instead the first image file it can find in that folder.
C) If in the same folder described in 'B' above I select a PDF file and at least one other file of any type, the viewer will display the PDF fine (and will be willing to navigate to other selected files, not that this is relevant).
Win7/SP1/x64, DOpus 12.8-12.9. PDF viewing per say has no issues. That said, I admit that this is all happening on a system 'contaminated' by several PDF applications, a bunch of Preview Handlers, and what not. Nevertheless, I hope that this will be reproducible on your end.