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Either delete or rename this file

/Library/Internet Plug-Ins/AdobePDFViewer.plugin
 
have you restarted Safari?

are you sure you went into the /Library rather than Users/username/Library/Internet Plug-Ins.

It might be in there for you.
 
If you open Adobe Acrobat you can go into the preferences and under the Internet pane, you will see at the top "Display PDF using browser" and then a drop down of programs. Probably best to close safari down first, pick preview in adobe and then reopen safari.
 
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