Viewer tries to open PDF files with Metro app as default

I'm currently trying out the new Windows 10 preview and have an issue with the Opus viewer trying to open a PDF file as well as the default 'metro' app. Now before anyone jumps down my throat that I'm using an unfinished OS, I know. However, this behaviour occurs on Windows 8 & 8.1 as well. If I install a separate PDF viewer program (SumatraPDF) opus behaves as expected. Could this be something to do with Windows file associations rather than Opus's file associations?

One thing I have just noticed is that the filename displayed on the metro app isn't the same as the one Opus's viewer tried to access even though the metro app has opened the correct PDF. Weird!

If you don't have a PDF preview handler installed then Opus will try to view PDF files via Internet Explorer, which might spawn them into the default PDF app.

You need to install a PDF viewer that includes a preview handler (or ActiveX viewer, or IE plugin) to view PDF files in the viewer pane.

Oh OK, thanks. As a product suggestion, could Opus include a PDF viewer as the Portable Document Format is quite ubiquitous?

There are lots of PDF viewers, most free to download (but not free to licence with other software) and everyone has their own preference. If we included a PDF viewer it would probably increase the price of Opus for something a lot of people would replace.

I am not sure there actually is a suitable PDF viewer we could license, either.

It is a shame Microsoft built one into Windows but only for the misguided Metro side of things, apparently forgetting to make it work with their own desktop file manager. If tge Microsoft PDF viewer were made to work with the File Explorer viewer pane, it would work in Opus as well.