For a long time now you've had the choice of viewing PDFs in Safari either with the built-in support of Preview or using Adobe Reader/Acrobat Pro.
With Snow Leopard it would appear that the Adobe Reader/Acrobat Pro solution doesn't work. It does work, just not yet in 64bit mode which is what Safari defaults to.
If you need/want to view PDFs using Reader or Acrobat Pro, then do a Get Info on the Safari app and enable the 32bit check box. Now when you launch Safari the Reader plug-in will work.
If you're wondering why would you want to do this? For most consumers this is a non-issue. Preview is fine. However, many of the business implementations of PDF forms out there require Reader or Acrobat to approve or fill out the forms in the browser. So in that case you would need the Reader/Acrobat plug-in working in Safari instead of Preview. I got bit by this a few days ago. Now I have a work around until Acrobat is 64bit.
You can never go wrong by doing the right thing.
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