To combine multiple PDF files:
Open each PDF file in Preview and show page thumbnails in Preview’s sidebar (choose menu option View > Thumbnails).
Choose any one of the PDF files opened in Preview as the starting point of the combined document.
In one of the other Preview windows select the PDF page thumbnails so that they are highlighted and then simply drag and drop them onto the PDF thumbnails of the combined document. Rearrange the thumbnail order by dragging and dropping within the thumbnails sidebar of the combined document, as required.
Repeat step 3. to add pages from other PDF files.
If required, rotate any of the PDF pages between portrait and landscape by selecting their thumbnails and choosing the Rotate Left or Rotate Right command on Preview’s Tools menu.
Select all of the PDF thumbnails in the sidebar of the combined document, and create a new PDF file from them by following these steps:
a. Choose Preview menu option File > Print.
b. Click the PDF button and choose the drop-down menu option Save as PDF.
c. Set the filename and folder location and click the Save button to finish.