Web pages change or may even disappear with time. Thus if you would like to preserve a web page forever, you should either need to download that page to your computer (and put it on Dropbox) or you could use a web archiving service that will safely store a copy of that page on their own servers, permanently.
There are quite a few ways to save web pages permanently and your choice of the tool will depend on the kind of web content that you are trying to archive.