ReaddleDocs has been updated to version 4.0 and is now called Documents by Readdle. Currently the app is free. It is not clear if the price will stay Free, so you may want to get it while you can.
The app serves many purposes including: use as a media player, managing files, viewing and editing documents. The app opens many file formats including .pages, .keynote, .numbers, .doc/.docx, .xls/.xlsx, .ppt/.pptx; .odp, .ods,. odt, .rtf, .rtfd, .txt, .html, .htm, .webarchive. However, you may not be able to edit those files.
The app allows you to read, listen, view, download and annotate many files on your iPad. Of course, you can sync files with Dropbox, Box and iCloud so you can make a modification on your iPad and view the change on another device.
You can watch a brief video to see how the app can be used below.
You can learn more about this app from the developer.