Weird one this. My copy of Opus has forgotten how to preview PDF files. Instead of showing the contents of the file, Opus either shows a PDF logo or selection of either HEX or meaningful text.
My PDF reader is Adober Reader DC, Opus is up to date.
Please go to Preferences / Viewer / Plugins, select the ActiveX + Preview + Office + Web plugin, and click Configure.
A dialog will open with a list of external viewers. Click the Defaults button in the bottom-left, then click OK.
That may be enough to fix things on its own. If not, go back in there and see which PDF-related viewers are in there. There may be an Adobe or other Preview Handlers in the top part of the list which doesn't have the .pdf extension assigned to it. You could try adding the extension, or moving it between viewers if there is more than one PDF preview handler installed, to find one which works.
You can also try adding .pdf to the Internet Explorer 32-bit line in there, which will make PDF viewing work if an IE plugin is installed on your system. This used to be there by default until recent versions of Opus, and may have been what you were relying on before; however, it isn't the preferred method these days, and should not be needed if you have PDF viewing working in File Explorer.