How to easily Combine two PDFs
3 Steps to combine PDFS in MAC for FREE!
1.Open a the PDF files in Preview (freeware).
2. Choose View > Thumbnails to show page thumbnails in the sidebar.
To indicate where to insert the other document, select a page thumbnail. Choose Edit > Insert > Page from File,* select the PDF that you want to add, then click Open. AND, you can just click and drag as shown below.
*Crtl A to select all thumbnails. 
3. Last, save, choose File > Export as PDF.
Easy and Simple!