PDF Assemble allows you to merge, split and re-arrange PDF format documents using a drag and drop interface.
Whilst the core functionality of this program is available in the Preview application that comes with every Mac, PDF Assemble adds a little more such as splitting documents and images of user defined sizes. It allows for a more efficient workflow because, unlike with Preview, all operations are carried out within a single window.
The program is available for purchase on the Apple App Store. Click on the above button to view it there.