One of the great features of Preview Version 4.2 (469.5) in Mac OS X 10.5.x was the ability to create one PDF out of many.  Here’s how the feature worked (using single page PDFs – this is important as I’ll discuss in a minute).

1. Open a PDF.
2. Expose the Sidebar.
3. Drag the PDFs you want to include in the Sidebar area.
   Note A: Each of the PDFs become page 2, page 3, and so on.
4. Save or Save As the PDF.  Voilà – one PDF.

For Snow Leopard 10.6.0, Apple changed the way this feature works in Preview Version 5.0 (501) and made it more cumbersome.  Here’s how it’s done in Snow Leopard:

1. Open a PDF.
2. Expose the Sidebar.
3. Drag the PDFs you want to include in the Sidebar area.
   Note B: The PDFs all retain their name in the Sidebar (contrast with Note A)
4. Select all but one of the PDFs in the Sidebar.
5. Drag and drop the selection into the unselected PDF recipient.
   Note C: You only get one chance at this or you’ll need to start over.  You can only drop one set of PDFs onto the recipient.
   Note D: After doing so, the recipient icon changes to include a Binding Comb on the left edge.  Moving the arrow cursor over the combined PDF exposes a circle-left-turn arrow on the right edge.
6. Save or Save As the PDF.  Voilà – one PDF.

Digging Deeper

Examining the problem further I’ve realized Apple has changed the Sidebar behavior even more.  If you open a multi-page PDF, the first page has the Binding Comb with a clear cover page opened and a circle-right-turn arrow with all the subsequent pages on display below.  Clicking the circle-right-turn arrow rolls up all the pages and closes the clear cover in a nice animation.  It appears they’re trying out a bookshelf metaphor and I’m not sure why.

Other observations:

• My Note C is a little disingenuous.  When a mutli-page PDF is open you can drag and drop more pages into it.  But why didn’t they automatically expand it when I did it earlier?  That might have given me a clue as to what was going on.  Especially since I was used to the old behavior.
• When a book is open, there is nothing visually tying the pages together.  There is no indentation.  There is no gray rectangle around all the pages.  You only get that sense if you more pages around or add a PDF not in the document.
• You can change the order in the Sidebar of books/pages (files) but I don’t understand why – you can’t save this view of the Sidebar.

Update 12/9 This procedure will not work if the creator of the PDF has password protected the document.