How to merge two pdfs into one in preview in MacOS

In order to merge two pdfs into one, follow the below steps:

SUMMARY: To combine two separate PDF files into one document you need to drag the new pdf ON TOP OF an existing thumbnail until the double grey border appears – then it will merge the two pdfs together. (You can then save the new merged pdf.) If you drag it into the sidebar but not on top of an existing page the new file will be added as an external link – not merged into the original pdf document. See these two pictures below to visualise the difference.

WRONG WAY: Drag the new pdf file under the existing one and they are not merged -it will only insert a thumbnail that links to the second pdf. (Single grey border)

RIGHT WAY: Drag the new pdf file on top of the existing one and they will merge into one – creating one pdf document out of the two. (Notice the double grey border.)

References :http://macintoshhowto.com/