When looking for software:
1. First, search MacUpdate.com
2. Then, search VersionTracker.com
3. Lastly, search Google.com
A simple search for "combine pdfs" at Macupdate.com gave me the aptly-titled...
Combine PDFs 1.0
This isn't a tech support issue; so, I'm moving it from the Genius Bar to the Third-Party Products forum.