In OS X Apple have again changed the way that 2 pdf files merge in the free OSX Preview App. To Merge 2 pdf files in OS X Mojave you need to drag the thumbnails of one pdf file into the thumbnail view of the second pdf file. You can’t drag a pdf file directly from the desktop into the new thumbnail window or the files will not merge.
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.
Apple’s built in pdf reader app called ‘Preview’ can merge 2 PDF files and edit PDF files. In macOS Sierra 10.12 all you need to do is drag the thumbnails that you want to merge from one PDF document to the other and then save the document.
Read on for some screenshots of what this looks like.
Apple have retained the ability to merge two PDF files with the  built in ‘preview’ app in OSX El Capitan.  As with  previous versions of OS X, there are 3 different ways that you can do this.  1. You can merge them into 1 document  in Preview as you are opening them. 2. You can open them as seperate documents in Preview then merge them by printing them to one pdf file. 3. You can open them as seperate documents in Preview and drag pages from one to the other.
Quite often I receive  a document to print double sided but I receive it as two single PDF files.  Other times I have two  documents that came from two different applications (for example one from pages and one from numbers)  and I want to print them back-to-back on one piece of paper using my double sided printer.  Here’s how to print to PDF files together using OS X Preview.
With every release of OS X Apple seems to slightly change the way to merge two pdf documents. I’ve previously written on how to merge 2 pdf files in Mountain Lion (10.8) and how to merge two pdf files in Lion or earlier. Here’s how to do it in OS X 10.9 Mavericks.Â
With Mountain Lion (OSX 10.8) Apple have made it even simpler to join two pdf files together – you just need to drag any pdf file into the thumbnail window of an existing pdf file and the two will be merged together!
Note: this is different for different versions of OS X.
For older versions of OS X (10.6-10.7)Â click here.
For OS X 10.9 Mavericks click here.
Thanks so much. Your explanation is brilliant. Easy to follow. Better than apple support itself. :)
OS X comes with a free application called ‘Preview’ that will open automatically whenever you click on a pdf file. Preview can edit and merge pdf files but the official Apple instructions are not quite right. Here’s how to do it depending on which version of OS X that you have.
You can merge PDF files using the built in ‘Preview’ application that comes for free with OSX, you don’t need 3rd party paid software.
The basic concept for merging 2 pdf files is the same for whatever version of OS X you have. You open the first file, view the page thumbnails, then drag the second file into the first file and save the new file. Â Exactly how this works varies depending on which version of OS X that you have. Apple have changed the details a few times. Sometimes there’s a green plus sign to indicate the merge. Sometimes there’s a grey border. If you don’t get it right the 2 files will not merge. Â You need to read the directions for the exact OS version that you have. (To find out which version of OS X you have just go to the Apple Menu and click ‘about this mac.’)
Please click on the version of OS X that you have to find out how to merge a pdf file:
How to merge PDF files in High Sierra (10.13)Â (2018)
How to merge PDF files in El Capitan (10.11)Â (2015)
How to merge PDF files in Yosemite (10.10)Â (2014)
How to merge PDF files in Mavericks 10.9Â (2013)
How to merge PDF files in Mountain Lion 10.8Â (2012)
For all versions of OS X prior to Mountain Lion (10.7 and earlier) Â just follow the instructions below.