PDF is a Portable Document Format, which is a popular format to represent documents and pass it over the internet. You can merge different PDF documents to obtain a single document. You can do this on your Mac, using the preview option. Preview does a lot more than the name suggests. You can actually merge PDF's and also rearrange pages inside the documents. There are some changes in the functionality in preview feature between the leopard and snow leopard versions. To use this first you will need individual PDF documents. Open it and select the view, side bar option to view the PDF pages in the side bar. Now open an other document. From the side bar drag the second document on to the first document till a plus sign appears. Now release the document and both the documents will be merged. This video shows how to merge PDFs using Preview on your Mac.