The most convenient way to create links to files in Dropbox is directly from your computer. However, with a couple of additional steps, you can get a link from the Dropbox web interface.
When viewing the page for which you want to create a link, you cannot use the one that shows in the address bar. This link would require students to sign into Dropbox. In particular, do not use a link with the word "preview" in the URL. (See example below.)
After you view the folder with the file in Dropbox, follow these steps:
The link is now in the clipboard of your computer, ready to be pasted into a link in Canvas or elsewhere.