No pdf view in viewer pane

Do you have antivirus as well as a firewall? Firewalls usually block network activity; antivirus is usually what would be blocking programs from opening documents.

Note that some antivirus software has multiple exclusion lists for different things. For example Windows Defender has separate exclusion lists for its virus scanning and privacy guard features, both of which can cause problems.

12.25 added an explicit error message to help people know which PDF viewers to install, but nothing else changed related to how PDF viewers work.

This is copied from the error message supplied by DOpus. I have Adobe Acrobat Reader DC 64 bit, which is the pdf reader that is used for Windows. This works fine on my desktop machine, but not on this one.
I also have Adobe Reader Touch, but that isn't as good as the above reader. Microsoft Edge works, but the Adobe Reader is better. None of these readers, when selected as default pdf reader, are working with DOpus as a previewer. I can turn off all sorts of settings with Norton 360 - temporarily - and that doesn't help. The Antivirus component does not block programs. That feature is in the firewall.
It used to work on this computer. I don't know what has changed.
Previews work fine with File Explorer, but I prefer DOpus for most functions.
The thing I like about previewing pdfs, when it works, is that you can rename them without having to close them first.

The only other thing I can think of is that something is preventing PDFs from being copied to your temp folder, or preventing them being opened when in your temp folder.

Opus will always copy PDFs to the temp folder before opening the copy, because too many PDF viewers lock the file while displaying them (even though it's against the Preview Handler API specifications to do so).

Type /temp into the path field in Opus to locate the temp folder, then see if you can copy a PDF to the folder and then display it, from that folder, within File Explorer's viewer. If that fails then it's likely what's going on.

I found the temp folder in DOpus as above and copied a pdf into it. I then copied that /temp path into File Explorer and thus located that temp folder. The pdf, as before, previewed in File Explorer, but not in DOpus.

Try a different PDF viewer in that case.

I have 3 PDF viewers on my laptop. They all work with PDF files on this computer. I.e. if I double click on anty PDF file on this computer, they open and display the file.
Within File Explorer, with each of the viewers as default for the computer, the preview function works.
With Directory Opus, with each of the viewers as default, the preview function does NOT work.
I don't think the problem is with my choice of PDF viewers.
Adobe PDF for Vista
My computer and DOpus are 64 bit. Is there a problem with the preview handlers being out of date in DOpus?

The preview handlers are not part of Opus, they're part of the PDF viewers.

You say the same ones work with your other computer but not on your laptop. Something on the laptop must be blocking Opus from opening PDF documents. That the only thing I can think of.

You could make a process monitor log of what happens which may reveal something:

I zipped the log file and moved it to OneDrive. I clicked on private message. In OneDrive I made a link and copied it and pasted that in the message. This is all new to me, so I hope it worked.
The icons in the toolbar are different. I clicked on Show File System Activity, which was fourth from the right.

It looks like the Process Monitor log only contains registry events, and it's the filesystem ones that we really need.

I've updated the instructions to match the new icons which Process Monitor added recently:

It's working now. What's changed? I don't know.
Here's a thought. Sometimes I don't shutdown my computer. I just put it to sleep by closing the lid.
Yesterday I had occasion to restart the computer a couple of times.
And now it works today.
Quote from The IT Crowd: Have you tried turning it off and on again? Roy, as the IT guy starts all conversations with this.

Oh, and thanks for your help.

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FYI for anyone using Adobe Acrobat 2020.

After installing this version of Acrobat it wasn't showing the PDF contents in the viewer. I checked Viewer preferences and everything looked normal. I had to re-open Acrobat, register/sign-in (which is what they require now) and once that was all done I re-opened DOPUS and the PDFs displayed in the viewer.

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