The “Preview.app” that comes standard with the Mac can do a lot of things, but I didn’t think it could even integrate PDFs. It’s easy to use. It was an eye-opener, so here’s a note.
Open the multiple PDFs you want to merge.
Here is the process of opening the files 001.pdf and 002.pdf. As you can see, there are two pdf files open on the right side.
Drag and drop the file you want to merge
Drag and drop one PDF file to the other. I wasn’t able to take a screen capture, but you’ll see a green + sign.
If you add it, the PDF file will look like a book. (I don’t think it will change for PDFs that already have multiple pages.)
Save.
Just press command+S to save the file. It’s easy.