When it comes to converting files Zamzar has been my hands-down favorite for years. It is reliable and versatile and you can convert everything from youtube videos to word documents. Zamzar's only drawback is that it can sometimes be slow and you have to wait for the new file to be sent to your e-mail.
So lately I have been using online-ConVert.com. It will convert nearly any file and if the conversion is going to take longer than 30 seconds or a minute you get a nice message saying, "It seems that the conversion of your file takes a while....If you want you can enter your email address below to receive a notification email when your file is ready to download." However, I have just been waiting on the site. It has been quite speedy.
Try it to convert that paper that your student wrote in Open Office, or extract an mp3 from the youtube of your kid singing.
In addition to the download link you get a handy QR code, so you can easily add any file to your phone! Plus, you can give the code to your students so they can access the file anytime. Brilliant.