I will assume that the error was on the iPad, not the PC, for my answer. You need to convert the video to a format that the iPad can play, or get an app that can play that format. You can't just change the extension name, it still won't work! If you can't see the extension on your PC, go to dropbox's website and look at it there. If that doesn't work, go into Windows Explorer, click on TOOLS > FOLDER OPTIONS > VIEW (tab) in the ADVANCED SETTINGS box scroll down to the entry that says HIDE EXTENSIONS FOR KNOWN FILE TYPES, uncheck that box and click the APPLY TO FOLDERS button (you should do this anyway, it was stupid of MS to default this to checked). Now you should be able to see your file extensions in Windows Explorer. Once you find out what the extension is, AVI, mp4, or what ever, check the app store for an app that will play that format. VLC and OPlayHD are favorites of many. To convert on the PC, go to google and search for iPad video converters. There are many, many programs available from paid to free, your choice. But, read the reviews before buying, many are also hard to use.
