If I try to delete a PDF file with that is being displayed in the Viewer Pane, I get a system message indicating that the file cannot be deleted because it is open in Directory Opus. At that point Directory Opus seems to have hung. I can't switch to a different file of any type to view, close the Viewer Pane, or close Directoy Opus. Takes killing the program from Task Manager to recover. In my case I had the Folder Tree pane open and two folder panes open, as well as the Viewer Pane.
If you go to Preferences / Viewer / Plugins, select the ActiveX + Preview + Office + Web plugin, click Configure and then find the PDF viewer in the list, try selecting it and then turning on the Open Temporary Copies option on the right.
It is using "Adobe PDF for Vista (32-bit)". If it makes a difference, I am on 64-bit Windows 10.
There is some other scenario that is causing the problem. I just tried copying copying then deleting several PDF files to make sure I could duplicate the problem before trying you suggestion. They all deleted without any problem. I have been working with PDF files a lot lately. I will try to determine the exact scenario that causes the problem.