On Mac OS X 10.10 with Safari 8, PDFs do not open using the Acrobat or Reader PDF browser plug-in. When opening a PDF in a browser, Safari's PDF Preview is the default PDF viewer.

In Safari 7, opening a PDF prompts you to enable to Adobe's PDF plug-in. In Safari 8, PDFs open in Preview automatically. You now must explicitly set the Adobe plug-in as the default PDF viewer.

Solution

Configure Safari 8 to use Adobe's plug-in:

1. Choose Safari > Preferences > Security > Internet plug-ins.
2. Click Website Settings. 
3. Select Adobe Reader. 
4. Navigate to When Visiting Other Website. 
5. Change to Always Allow.
6. Choose Done.