Apple’s built in pdf reader app called ‘Preview’ can merge 2 PDF files and edit PDF files. It used to be tricky to merge two PDF files ( read some of my old articles here) but in macOS Sierra 10.12 it’s easier than ever. 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. You don’t need to buy any extra software. It is built right into OS X.
Read on for some screenshots of what this looks like.