How do you quickly combine multiple PDF files into a single document?

1. To do so, select your PDF, then 'File,' then 'Duplicate' from the drop-down menu.
   
2. Open the preview window.
3. Select one of the PDFs you wish to combine and open it. In this tutorial, we'll combine two of them.
4. Click 'View' in your PDF file.
5. Then, in the sidebar, click 'Thumbnail' to see thumbnails.
6. In Preview, drag another PDF from your desktop. The two documents will be combined as a result of this.
7. You can put them in whatever sequence you like, including in front of, behind, or between existing pages. Rearrange the thumbnails by dragging them.

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