Many of my tasks, at Orbeon or personal, involve PDF documents:

• Sending invoices to customers (no way we are sending Word or Excel documents!)
• Archiving, viewing and searching documents scanned with the ScanSnap scanner
• Viewing faxes received by email
• Saving online articles for reference, further reading or printing (until we get good e-paper I prefer to read long articles on good old dead trees)
  

I was already using PDF a lot on Windows, but that required installing a free viewing tool (Adobe Reader) and a paying PDF printing tool (Adobe Acrobat).

When I switched to the Mac back in 2006, I appreciated right away the built-in ability to view and print PDF documents without buying a third-party tool. But Leopard adds even more to the mix:

• The new Quick Look feature in the Finder and Mail is not perfect, but excellent nonetheless: just press the space bar (Finder) or a button (Mail) and you have a super-fast preview of almost any document or attachment, including PDF. You can even scroll down multi-page documents and keep the preview open while navigating to other documents. (To be fair, OS X should have had this for years but better late than never.)
• The new Preview allows you to insert, delete, rotate, and reorganize pages in a PDF document and save the result.
• The much improved Spotlight automatically indexes PDF documents for (almost) instant content search.

Windows XP has nothing of the sort built-in. Vista has XPS, which sounds cool but, no matter how good it may be, is today nowhere as widespread as PDF and that alone does not make it a viable option for me at the moment. Leopard definitely wins this one.