If you want a tool with a simple GUI, try pdfshuffler. It allows for merging of PDFs as well as rearranging and deleting pages. For batch processing and/or more complicated tasks, pdftk is of course more powerful.
I tried using it in my Ubuntu 14.04, doesn’t work for me, which just gives the following alert box when I tried to export.
But I tried pdftk command line tool, which works perfectly. Here you go…
You can install it by opening terminal and typing: