Do you mean how to combine two pdf files? You can open one pdf file making thumbnail shown, and then just drag the second pdf to the shown thumbnail zone. You should see the green "+" sign, release and you are done. After the second pdf file is added, you can drag one or multiple pages to adjust the order.
@@tiezheng1020 I just figured it out! I thought you said Command C then Command B. I tried it 5 times then figured out you said command C then command V!. Thanks for your help!!