Since we authors, published or self-published, often have to deal with a PDF file, be it for sending our work to publishers, to have our paperbacks printed or prepared for sale in print-on-demand, I would like to recommend the free PDF24 tool today.
I mainly use it to put transparent overlays over the pages, although the program is capable of much, much more. These overlays are purely ornamental, at least in my case, but they help stand out a bit from the crowd of other publications, add individuality to the printed book, and add to the perceived value.
There are still other possible uses for PDF24, there is an initial learning curve but it is fairly flat and the program is completely free.
