merge pdf in preview

Open the side bar in Preview. Drag your additional PDF on top of the existing PDF to combine.

Preview>View>Thumbnails or

If it is a multi-page pdf, click the disclosure triangle in next to the destination to open up all the pages. Then, you can drag the additional pdfs into any page location you want.

http://macintoshhowto.com/beginners/how-to-merge-two-or-more-pdf-files-in-mounta in-lion.html

Or just use Auomator.

Here's an example: