The Preview.app in 10.5 and newer has many new features. One that appears to be undocumented, however, is the creation (merging) of multi-page PDF documents. It’s simple:
Use Preview to open up the two PDFs you would like to merge. Choose View Â» Show Sidebar (or click the Sidebar button). Make sure both PDFs are visible on the screen at the same time. When the Sidebar pops out, you will see a graphical representation of the pages in your PDF document. Simply drag the page, or pages (use Comand to select multiple pages) from the Sidebar of one PDF to the Sidebar of another. You have now merged pages from two separate PDF documents.
Two related hints:
You can use the proxy icon in the PDF window’s title bar if you want to copy one entire PDF to another PDF. Just drag the proxy from the title bar of one PDF to the Sidebar of another.
You can copy some types of files directly from the Finder to a new PDF using Preview’s New From Clipboard (Command-N) feature. Go to the Finder, select an image file such as JPEG, and copy it (Command-C). Now go to Preview and do Command-N. Your JPEG file will show up as a new PDF document. This seems to work for many graphic types, even the Adobe PSD Photoshop format.
Credit for this very useful tip goes to MacOSXHints.com