Apple - How can I combine two PDFs in Preview?

First, you can only merge pdf if they are not protected, if this is the okay :

1. Open both pdf
2. Enable the sidebar in each windows of Preview, then select "Thumbnails" view
3. Drag & drop the cover (inside the sidebar) of one pdf into the sidebar of the other pdf.

With this method, you can completely merge two pdf or selectively drag & drop the pages you want to merge to create a completely custom new pdf.

You can also extract pages from one pdf still using the same manipulation, but drag & drop to the Desktop.

I know you asked for 'Preview' solutions, but in case you're interested in a command-line tool:

alias pdfjoin='/System/Library/Automator/Combine\ PDF\ Pages.action/Contents/Resources/join.py'

Then you can do things like

pdfjoin -o out.pdf file1.pdf file2.pdf ...

Here's a detailed explanation of how to use Automator. It looks complicated but it only took me a couple of minutes to reproduce it on my computer...

http://www.documentsnap.com/how-to-combine-pdf-files-in-mac-osx-using-automator-to-make-a-service/