I too have had Safari quit and/or hang when trying to view PDFs. I recently loaded Adobe Acrobat Professional 7.0 (CS2 version) so I was suspicious it might be the problem.
Here was how I fixed it:
1) Download latest Adobe Acrobat Reader (9.0 at this time) & install
2) Quit Safari
3) Open Adobe Acrobat Professional
4) Select Acrobat:Preferences:Internet
5) Click the top check box: "Display PDF in browser using:"
6) Select Acrobat Reader in the list box
7) Open Safari & navigate to a PDF link or try this one -
http://www.irs.gov/pub/irs-pdf/fw4.pdf
8) Reader may ask you to input your password for access. Enter Password.
9) Document should open now.