Apple has a guide on Use Preview to combine PDFs on your Mac.
This guide works on the latest macOS Sierra, I have not tried previous versions:
- Create a new copy of the first PDF-document you which to merge into (because Preview saves the result in the source).
- Open the copy of the first PDF-document in Preview.
- Open Finder and drag the second document below the last page of the first document.
- Close Preview. It will automatically save the document with the edits you made.