Suddenly I can not see pdf files in the viewer pain..just code and numbers etc. Some files do show an image but not clearly rendered.
What do I check?
I used opus for several version and no trouble until now.
Win 7, 64 bit, opus 12.. I use adobe pro XI and chrome browsers
Thanks for the help.
Normally I see pdf and even scroll through pages.
G S K
There is a setting you could check, -> preferences -> viewers -> plugins -> ActiveX + Preview + Office + Web.
In the plugin config which Abr points to above, see if there are any PDF viewers in the preview handlers section at the top. There may be one there without the pdf extension assigned to it, if another PDF tool has taken over its registry settings. If so, you can add the extension there to tell Opus to override the registry settings.
If that doesn't apply, try adding .pdf to the Internet Explorer (32-bit) line lower down the list. This used to be the default but was changed recently as it causes pop-up download prompts for people who don't have an IE plugin for PDF installed, and newer PDF viewers work via preview handlers rather than IE plugins.
If it still doesn't work, undo that config change and then (re)install a PDF viewer which includes a preview handler. Examples include Adobe Reader and PDF-XChange. (Adobe Acrobat does not include a viewer. Adobe have chosen to keep their PDF editor and viewer as separate downloads/installations.)
abr.. thank you. Yes I checked and those options are checked on.
leo..thanks. I have adobe viewer on computer.Here are the current plugins..Where next?
Select ActiveX + Preview + Office + Web and click Configure. My post above then applies to the list you'll see. (If you get stuck, please post a screenshot of it.)
Leo, thank you so much. That resolved the problem. Opus is great. And your timely response was good before I leave for a long teaching tour. 
Perhaps you have a suggestion for one other thing. Very frequently I lose explorer shell..can't get the main programs list to respond for example.
When I use CC cleaner and scan registry I get all dopus problems. I fix. Then the normal function returns. But it happens a lot. Is there some dopus fix? THanks
The best advice we can give you is to not use CC Cleaner. Registry cleaners are of very little use and generally cause far more problems than they could ever solve.
Stop giving advices! Registry Cleaner, TuneUp and all others are very helpful, because I can earn money for every installation destroyed by these great tools. 
I'm experiencing the same problem as the OP. I followed Leo's instructions to add "pdf" to the list extensions for IE 32 bit. The result was not good. PDFs in the viewer pane would give a "file download" popup and show an IE page "navigation to the webpage was cancelled".
Next I'll try to make sense of the other advice about installing PDF preview handler. I admit I find this topic confusing.
The Internet Explorer option for PDF doesn't make sense on most machines these days.
You just need to install a PDF preview handler. (e.g. Adobe Reader, PDF-XChange, FoxIt, or Sumatra with the appropriate option selected during installation.)
Make sure it's working in File Explorer's viewer if it isn't working in Opus. It should work in both, but if it works in neither then that tells you there either isn't a PDF preview handler installed or something has messed up its registry settings and the preview handler may need reinstalling.
Thank you, Leo. I installed PDF-XChange with default settings (all components) and PDFs in DOpus magically worked right away.
I have DOpus 12.24 and Windows 10 64 bit. The pdf previewer works on my desktop computer, but not this Dell laptop. I have Acrobat Reader DC, Acrobat Reader Touch and Microsoft Edge. They all work in Explorer, but not DOpus.
In Preferences, Viewer, Plugins, Active X .... Configure, I have checked Adobe PDF for Vista (32-bit). Does DOpus need updating here?
I have tried Download More Plugins, but there is nothing useful there.
I feel that DOpus on this computer used to preview PDFs, but it doesn't now.
Please advise.
Thanks
John
Make sure the PDF extension is also assigned to it.
On a new system, I found Adobe Reader DC doesn't work (in Opus or Explorer) until the standalone version of it has been launched once and the licence pop-up agreed to. But, beyond that, it just works once installed.
If it's working in Explorer but not Opus, my suspicion is you have antivirus/privacy software which is blocking dopus.exe, docsvw32.exe or docsvw64.exe from opening PDF files or from loading the Adobe components. If you're using Defender, see here; otherwise, try excluding the same processes from the antivirus/privacy software that you use.
Thanks. Tried that in Norton firewall by allowing those files. Didn't help. Restarted the computer and that didn't work either.
Did you want a screenshot? If so, how do I do that in this forum? Is it the Upload button?
You can just drag a saved image, such as a screenshot, into your post.
Thanks. Here is a screenshot now, in case it helps.
Try reinstalling Adobe Reader. Failing that, I would try another PDF viewer.
I've tried 3 different pdf readers in default apps, and none of them work with DOpus. They all work with Explorer.
It's almost certainly antivirus or anti-ransomware software blocking Opus from opening PDF files if none of the different viewers work.
I just temporarily turned my firewall off and it didn't fix it. Now I have 12.25 and there is the error message:
This PDF document could not be displayed.
This could mean one of the following:
Repairing or (re-)installing a PDF viewer is recommended if:
PDF viewers with preview handlers include:
