Mac OS X 10.5, or Leopard, gave the Preview app some serious productivity powers, but at least one feature, PDF combining, works a bit differently in Snow Leopard (10.6). PC World explains the subtle but significant change.
In Leopard, adding pages from one PDF into another was a matter of grabbing those pages’ thumbnails from the Preview sidebar and dropping them in above or below the thumbnails in another document’s sidebar. Not so in Snow Leopard:[imgclear]
The trick is to drag thumbnails on top of a thumbnail in the document you’re dragging to. This combines the pages. If you want to rearrange the pages, just drag them where you like—they’ll stay within the combined document. Save your document and you’re done.
Nice to know, especially if it helps you avoid installing yet another unnecessary PDF tool in your system. A bit more about what to do if you’ve got multiple documents open in Preview is explained at the link.
Combining PDF Pages in Snow Leopard [PC World]