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At a guess your PDF editing program has "taken over" as the default PDF application for your operating system. You don't say what OS you are using (Windows x, Linux, or MacOS). But basically you need to restore the file association back to the original PDF viewer you were using (be it Adobe or Foxit or whatever PDF viewer was your preference).
If you want to keep the file association to your editor but use a different application to view in calibre then you could use the Open With plugin. But I suspect your solution you really want is to get the pdf viewer as the default application regardless of whether you are in calibre or not. And the instructions for that as I say differ depending on your OS.
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