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.
I'm using "Foxit PDF (32-bit)" viewer. But even if I disable the whole ActiveX +... viewer item, and PDFs start to be viewed with Hex viewer (as a binary dump), I am still seeing the same behaviour: an image (if present) is opened instead of the single selected PDF.
But if you can't reproduce it, don't worry about it. I'll double-check on a squeaky-clean VM one of these days. Maybe it's something peculiar with my install.
I have reproduced this issue on a very clean VM with DOpus 12.8 (with all Shell integrations declined but otherwise zero changes to settings) and 12.9.1 Beta (with all default setup wizard suggestions accepted).
The OS is still Win7/SP1/x64, and it's essentially untouched (aside from some updates and networking changes - nothing I can think of that could possibly impact the issue at hand). In particular, Win7 has never seen a single PDF program, so it knows nothing about PDFs.
So, while my OP description still stands, let me reiterate:
Install DOpus and open a new Lister
Navigate to C:\Work\ (test folder does not seem to matter)
Place 2 files in that folder: a.pdf and b.png (names don't seem to matter either)
Focus a.pdf
Invoke the standalone viewer
The viewer opens and displays b.png
Note that a.pdf doesn't even have to be a real PDF. It can be a text file with "Hello, I'm a fake PDF" inside. What seems to matter is the extension - and the presence of at least one image file next to it.
(When a.pdf is viewed - if explicitly opened from the viewer or if viewer is invoked with both a and b files selected - it is displayed as plain text or hex dump, of course - since there are no PDF viewers in the system.)
Let me know if there's any additional digging that I can do on my side to help you reproduce this. Thanks!
I may be off-base, but I'm not sure what you describe matches my experience: if I ask DO to view an essentially unknown file (be it PDF or some totally random extension), the viewer normally opens that file just fine - in plain-text or hex-dump modes. This part may be DO-configuration-dependent though: I have shut down the clean PDF-agnostic VM, so I can't double-check this ATM. Or maybe I can check this differently...
Well, I performed the following experiment on a VM that can view PDFs: folder with 2 text files: 1st called a.lksjdlfkj (random extension) and 2nd called b.jpg (image). If I ask DO to view a.lksjdlfkj, it opens as expected. If I rename a.lksjdlfkj to a.pdf and ask DO to view it (in standalone viewer), the viewer displays b.jpg instead.
Plus, there's still that puzzling bit: if 2 or more *.pdf (or any) files are selected, invoking the viewer works as expected (regardless of the presence of any images in the same folder).
I have investigated this issue a bit further: Show AUTOFILELIST - the only 'version' of the command that exhibits the problem I have described above. Show LISTSIBLINGS (or Show with no arguments) works as expected - displaying a single selected PDF.