It is much easier to combine PDF files on Mac OS X, not like using a PDF merger on Windows computers, you can use the built-in app – Preview to merge PDF files to one.
Below are some tips to combine PDF files on Mac, both ways are compatible with OS X Mountain Lion, Lion and Snow Leopard.
First case: If you want to combine two PDF files:
Step 1, simply open the PDF file in Preview.
Step 2, open the thumbnail view (Shift-Command-D).
Step 3, drag a second pdf file ON TOP OF the existing page thumbnail. NOTE: The second opend PDF file must be on top of the first opened thumbnail. If you drag it into the sidebar but not on top of an existing page the new file will be added as an external link – not merged into the original pdf document.
Step 4, combine two PDF files into one.
And if you want to merge more PDF files into one on your Mac, below method would be better.
Second case: when you have to combine more than two PDF files:
Step1. Open the PDF documents that you want to combine in Preview.
Step2. Show the sidebar in each Preview window. If the sidebar is not showing, click the Sidebar button in the Preview window’s toolbar, or choose View > Sidebar > Show Sidebar.
Step3. Verify that the sidebar is configured to show thumbnails (this the default view). If the sidebar is not showing thumbnails, choose View > Sidebar > Thumbnails.
Step4. Drag the thumbnail of one PDF (source) over of the thumbnail of the other PDF (destination) and release the mouse or trackpad button when the pointer shows a Green Circle with a plus symbol in it. This will copy all pages from the source PDF and add them to the end of the destination PDF.
Step5. If the pages in the destination PDF are shown in the wrong order, click the gray circle containing an arrow that appears over the PDF document’s thumbnail, so that the arrow points to the right, and drag the thumbnails of the individual pages into the desired order.
Step6. After the destination PDF contains all of the pages you want and the pages are in the correct order, you can select File > Save As to save a new PDF document containing all of the combined pages, or you can select File > Save to replace the original version of the destination PDF with the version containing the combined pages.