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.)