Go to the "Help" menu in Safari and select "Installed Plug-ins". This will open a new tab displaying information about all of the currently installed plug-ins. Find the entry for Adobe Acrobat. Below it will be a line that reads something like:

Adobe® Acrobat® Plug-in for Web Browsers — from file “AdobePDFViewer.plugin”.

Note the name of the file from that line.

On your hard drive you will find a Library folder. In that folder will be another folder called Internet Plug-Ins. Inside that folder you should find the file that was shown in the "Installed Plug-ins" tab. The full path should be /Library/Internet Plug-Ins/AdobePDFViewer.plugin. That file needs to be deleted. Dragging it to the trash will do the trick.

If you are not sure you want to delete the file, you can move it to another location. For instance, you could create a new folder named "Disabled Plug-Ins" in the "Installed Plug-Ins" folder, and move the file to the new folder.

Then quit & relaunch Safari.

Adobe acknowledges this is a "known issue" and recommends this procedure.

1st thing to try

1. Open Firefox
2. Open Preferences (Firefox > Preferences, or Command + ,)
3. Select the Applications tab
4. Search for "pdf"
5. Change action to something else, then change it back to "Always Ask"

Verify this is set correctly

1. Open Finder
2. Search for .pdf
3. Right (Option) Click the first PDF that appears.
4. Click Get Info
5. Expand Open With (if it isn't already)
6. Change it to "Preview"
7. Click Change All button below the drop down menu
8. Accept the prompt.

For a visual representation: