Click ADVANCED-> MERGE -> MERGE MULTIPLE PDFS TOGETHER:

The following above will appear.

Click the ADD PDF button to select a PDF file or group of PDF files that you want to merge into a single PDF.   Click REMOVE PDF to remove the selected PDF from the list of PDFs to be merged.   Clicking the UP and DOWN arrows will move the selected PDF up or down in the list of PDF files.   The order of the PDFs in the list will determine the order in which the PDFs are appended to each other.   Click the MERGE PDFs button, you will be prompted as to where you want to save the merged PDF (the PDF that will be all the PDF files appended to each other in the order specified in the dialog window).   After you click SAVE, the PDF files will be merged together.   This may take a few minutes depending on how large the PDF files are and how many PDF files you are merging together.