You can merge up to 10 PDF documents into one by clicking the Merge PDF link, selecting the documents (from your computer), and clicking the Merge PDF button. When it's finished, you can download the and rename the new document.
Do you have one large PDF file that you'd like to split into smaller separate files? Just click the Split PDF link, select the original PDF, identify the page ranges for the new documents, then click the Split PDF button. When it's completed, you can download a zipped folder with all of the individual files.