Even on a desktop, editing PDF files is not an easy process, so PDF Slicer Free is a very useful phone app for streamlining rotation, splitting, combination, and reordering of PDF documents. While the interface and lack of in-app help can make it hard to find your way through the sheer volume of tools it offers, once you do, you'll have much greater control over the PDF files on your phone.
When you load PDF Splicer Free, you can load or create a new document. There are so many functions that it's hard to get your bearings at first. It wasn't immediately clear what everything did, so learning how to use the app could be mostly trial and error. However, in a few minutes we were able to download a PDF from Dropbox, load it into the app, and then split it into two separate PDF files. The same two PDF files could then be converted into a single file afterwards. Other functions like rotation of pages, deleting individual pages, and inserting new and blank pages all worked quite well, though again, after some work finding those functions. From start to finish, the app did everything we could think of to edit a PDF on your phone -- the only major drawback was the at times frustrating interface, which admittedly makes more sense once you've used it for a bit. Additionally, there is a packaged How-to video from the developers that can greatly expedite the setup process -- something we highly recommend for new users.
If you need an (ultimately) easy-to-use tool to edit your PDF documents on the go, consider PDF Splicer Free. While ad supported, the app still features all of its built-in tools, though there is a cover page with an ad if you don't upgrade to the paid version. After the initial learning curve, it is immensely powerful.