Go to Settings -> Search for Applications

Look for "Portable Document Format (PDF)"
Set Action to "Use Adobe Reader" --> you may need to browse for it ("use other") button
C:\program files (x86)\adobe\Reader 11.0\Reader\AcroRd32



Might also do the same with:
Adobe Acrobat Forms Document (ap...
Adobe Acrobat Forms Document (ap...
Adobe Acrobat XML Data Package File

Firefox also has a built in viewer, it doesn't always work the best, so it is preferred to use Reader.