For those who use Preview to combine PDF documents, Snow Leopard has added a twist to the process. In earlier versions of Preview, combining documents was simply a matter of dragging the individual pages from one document's sidebar to the other document's sidebar, and then rearranging the pages by dragging them up and down in the sidebar list of the combined document.
Preview now allows you to open more than one document in a single window -- you can have any combination of PDFs, JPEGs, and whatever else Preview can open, all open at once. When used in this manner, the individual pages from all the documents will be displayed in the sidebar.
In addition, Apple added a new metaphor in Preview: a multi-page PDF is now displayed in the sidebar to look like a comb-bound book. (Preview lets you 'close' the book and reduce the display to just the first page.) You can add pages to a multi-page PDF by dragging them into the sidebar, either into an open or closed book. As we did in the past, we can control the order of the pages by paying attention to where the insertion bar appears. Preview highlights an outline around the sidebar image to let you know you're adding pages to the PDF.
The trick to combining single-page documents in Preview is to know that when you drag a page into the sidebar, Preview assumes you want to combine the pages in one window. It doesn't combine the pages into one document, and displays the insertion bar only to indicate the order of the individual documents in the sidebar.
Instead of trying to drag a page before or after a single page, you now have to drag the page on top of the other page in the sidebar. Preview will then change the page image in the sidebar to the bound book (multi-page PDF) icon. You can then rearrange the pages in the PDF in the order you want.
What's neat is that you can combine any kind of document that Preview can open. Once combined, they can only be saved as a multi-page PDF.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090915223224601