usually, when you right click a file you can click "Properties" at the very bottom of the menu that opens.
In the properties window, towards the top there is usually a line that says "Opens With:" and a button that says "Change"
This opens a window with some selections of programs that can open the file (Recommended Programs are shown first, but you may have to click the arrow next to "Other Programs"). Select your PDF viewer (probably Adobe Reader) and click OK. Then click "OK" in the Properties window. This should make all your .pdf files open with adobe.
Alternately, you could try re-downloading and installing Adobe pdf
https://get.adobe.com/reader/otherversions/