Merge Multiple PDFs in Preview

You don’t need Adobe Acrobat to merge multiple PDFs into one document — just Apple’s Preview application. Here’s how:

Open one of the PDFs in Preview, and display the sidebar to reveal its thumbnails (click the Sidebar button on the Preview toolbar or press Command-Shift-D).

Then just drag a PDF file (single or multiple page, of any dimensions), onto the sidebar at the location you want its pages to appear in the first PDF file. When you save that PDF file, it will include the other(s).