All you need it to run the software, drag and drop your PDF files into the window, and click the 'Merge' button. After few seconds your files will be combined into one PDF document.
After PDF joining, you are able to improve your document: to add the text of image watermarks, to set permissions, to edit metadata. You can also crop, rotate or delete the pages of PDF. The software is completely free, so download it right now. You will be amazed by simplicity and a pretty interface of Combine PDF software.