As I mentioned in this post Apple regularly change the way to merge a PDF document. Yet again it’s changed with High Sierra – for the better.  Now when you drag a pdf onto the thumbnail window of an existing pdf the two documents will ALWAYS merge. This make it easier than ever to merge 2 pdf documents. Here’s how to merge 2 documents in High Sierra.

Merging 2 pdf files in High Sierra

1. Open the first pdf Document in Preview by double clicking on the pdf file.
2. Select ‘view Thumbnails’ from the drop down menu in the Preview window so that you can see each page of your PDF document.
   

   Show Thumbnails to make it easier to merge a PDF file.

    

3. Make sure the little arrow in the top left is facing down so that you can see all the pages in the PDF file
   

   Make sure the triangle arrow is pointing down.

    

4. Now all you will need to do is go to your desktop, click onto the second pdf file and drag it onto the thumbnails. A little green plus sign will appear to show you that the file is being added to your pdf.
   

   In OSX 10.13 you can drop the new pdf anywhere, the green plus shows it will be merged.

5. Now save the pdf file and the new pdf file will be a part of the original pdf.
   

   Now you can save the merged pdf.

This is so easy! But these instructions are only for high Sierra 10.13.   It is slightly harder in earlier versions of OS X, click here for how to merge 2 pdf files in older versions of OS X.