There are a number of ways to do this but I find the easiest way is to open the PDF documents and drag the page thumbnails from the pages panel of one document into the pages panel of the other PDF document provided you are just moving a few pages.
If the PDF has multiple pages of you want o combine multiple PDFs go to:
File > Combine files into single PDF. Or you can use the Create button on the top left corner of the application window.
You cannot do this with Reader only Standard or Pro