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Are the PDF files associated with Acrobat or Reader? If not, open the application, go to its Preferences (used to be under Edit, might be under the Menu button now), then go to the General tab and click the "Select as Default PDF Handler" button at the bottom of the window.

May be that you have used a other app in the past.

@집착38216136t3ol 

I've moved your post to the Acrobat forum.

Jane