It is not to send the odd file to friends. I have been producing e-books, so most of them go to complete strangers who may not like the idea of having to download software into their computer to get the files. They don't mind geting a CD disk in PDF for Acrobat Reader and I have sent them all over the world (by airmail) with no problems.
I just took another look at Dropbox and although I am quite computer-literate for most things, I could not even get it to install at all. Maybe it is my Avast that is preventing it!