Users of Mac OS X 10.5 (Leopard) or greater can merge multiple PDF files into a single PDF file using the Preview application. This is a helpful tool when you have multiple scanned images saved in a PDF format or you simply want to consolidate research.
1. Open Preview (Go -> Applications -> Preview)
2. Click File -> Open and choose the first PDF
3. If the Sidebar is not displayed on the right side, click the Sidebar button along the Menu bar.
4. Open up the Finder and locate the PDF files you wish to add to the first PDF file.
5. Click and Drag the second PDF file into the Sidebar of the first PDF.
5a. You can also merge individual pages. To do so, open the second PDF file in Preview. Click and Drag individual pages from Sidebar to Sidebar.
6. Repeat step 5 for each PDF you wish to merge into the new PDF file.
7. Click File -> Save As to save the new PDF file
Note – This will only work with Mac OS X 10.5. This feature is not present in 10.4 or earlier.