Which PDF viewer were you using before?
If it's a modern viewer like recent versions of Adobe Reader or PDF-XChange, you may just need to repair or reinstall it.
If you're using a PDF viewer which only includes an Internet Explorer plugin, and not a modern Preview Handler, then recent updates to Opus will no longer use it by default. You can re-enable it via Preferences / Viewer / Viewer Plugins, select and configure the ActiveX + Preview + Office + Web plugin, and add .pdf to the Internet Explorer 32-bit line. (If that causes you to see prompts to save the file when you try to view it, undo it as it wasn't what was needed to make things work after all.)