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Workspace
Last updated 1/14/2015
3 Click the Enable or Disable button (it toggles depending on the status of the selected add-on):
Enable sets plugin to open PDFs in the browser.
Disable turns off the plugin so it does not open PDFs in the browser.
For more information, see the Mozilla Firefox help topic Using a PDF Reader Plug-in.
Apple Safari
Make Acrobat or Reader the default viewer for PDFs
1 In the Finder, select a PDF, and choose File > Get Info.
2 Click the arrow next to Open With to expose the product menu.
3 Choose either Adobe Acrobat or Adobe Reader from the application menu.
Very advise you to visit a site that has a lot of information on the topic interests you.
Also what in that case to do?