Q: How do I change my operating system's default application for opening PDFs?

Answer

To set a PDF viewer as the default on Mac OS X:

1. Select any PDF file from Finder. Control-click to open the menu.
   Result: A menu will open next to the file name.
   
   
   
   
2. Choose Get Info from the menu that opens.
   Result: A new window will open with information on that PDF file.
   
   
   
   
3. From the Open with: section in the new window that opens, select your preferred application, such as Adobe Reader or Preview.
   Result: That application will be selected.
   
   
   
   
4. Click the Change All button. In the subsequent dialog box, click Continue.
   Result: The application that you selected will be the location in which PDFs open.
   
   
   
   
   

To set a PDF viewer as the default on Windows

• If no other PDF viewer is installed, once Adobe Reader is installed, it becomes the default.
• To set a preferred PDF viewer as the default:
  • Windows 7:
    1. Follow the menu path Start > Default Programs > Associate a file type or protocol with a specific program
    2. Highlight .pdf, then click Change.
    3. Choose your preferred PDF viewer, such as Adobe Reader.
  • In Windows XP
    1. Follow the menu path Start > Control Panel > Folder Options > File Types.
  • In Windows 10
    See: Tech Minute's Instructions

See also:

http://pcsupport.about.com/od/fixtheproblem/ht/defprogramxp.htm