Welcome to TS!
Find a pdf on your drive and do a Command I on the document, (click once, then apple key-letter I) . Look for the triangle to the left of "Open With". Click on the triangle, and it'll show you the default application. Now click on the drop down arrows and choose the application you want it to open with. If the application you want is not in the list, choose "Other" and navigate to the application you want. The "Change All" button underneath will become "active". If you want to change all the pdfs to open with this choice, click it, and then close the Get Information window.
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