Steps to Combine multiple PDFs into One file on a mac:
1. Open the first PDF in "Preview"
2. In Preview Make sure side menu is viewed by clicking on little menu icon and clicking "Thumbnails.
3. Go back to Finder and select the remaining PDFs (Click on file hold "shift" on the keyboard, then click the last file to select multiple PDFs
4.Drag the selected files into Preview's sidebar menu - beneath the original pdf
5. Click "File"
6. Select "Export as PDF"
