Basically the exact opposite of this existing question How do I disable inline PDFs in Safari 5.1?.
I need to view my PDFs in-line, and with Safari 5.1 it's no longer working on either OSX 10.6 or 10.7.
If you install Adobe Reader when you open for the first time you will see this box, select yes.
If you already have it installed go to preferences and make sure "Display pdf in browser" is checked.
For what it's worth, on a new install of Lion, clicking on a PDF loads the file in Safari.
If you are saying that clicking on a PDF on your computer opens it in Safari, you can correct that behavior by selecting any PDF and clicking Command-I, or selecting Get Info under the Finder's File Menu.
In the information window that opens, you will see a drop down menu titled "Open With." Select Preview for PDFs, navigating to it in the Applications Folder if need be.
Then, beneath that, you will see "Use this application to open all documents like this"
Click on "Change All" and you're good to go—and this can be done to change any file's application binding.