You don’t need Acrobat Professional or any other thrid party pdf software. It’s built right into Mac OSX.
If you want to merge PDF Files, just open the first one in Preview. Second, open the Sidebar by Shift+Command+D or like this:
Third, drag and drop the PDF File you want to add right into the first one – it’s important that you drop it on top of the first one:
If you added sucessfully your second PDF File, an arrow will show up on the right side (see screenshot below). Just save your merged files. That’s it!