Using the DropBox app, select the file. When it finishes downloading use the share icon at the top right, and you will see what programs the file can be opened in. Pages will open a .txt file, as well as .doc and it's own format.
Pictures will only show a Save Photo option. These are save in the Saved Pictures album in the Photos app. Programs that can use photos usualy have thier own method to import photos. Almost all of them can do so from the photos stored in the Photos app.
PhotoShop Express opens up and asks you to select a photo. Tap it and you'll be able to select one from the albums in the Photos app.
If you need a photo in Pages you import it into a document using the Media icon (looks like a little picture, framed). Again, you choose from the photos stored in the Photos app.