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Just drag the proxy from the title bar of one PDF to the Sidebar of another.
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Simply drag the page, or pages (use Comand to select multiple pages) from the Sidebar of one PDF to the Sidebar of another. When the Sidebar pops out, you will see a graphical representation of the pages in your PDF document.
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Make sure both PDFs are visible on the screen at the same time. Choose View » Show Sidebar (or click the Sidebar button). Use Preview to open up the two PDFs you would like to merge. One that appears to be undocumented, however, is the creation (merging) of multi-page PDF documents. The new Preview.app in 10.5 has many new features.
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