You can invite other dropbox users to a folder but that gives them access to the entire folders contence, you can give any one access to the individual file through a direct link.
If you have it installed on your computer, make sure the file is in the drop box public folder (either way), then right click on it and there will be a drop box option on the menu (drop box has to be running to see it), on the sub menu there is a "copy public link" option which is the link to the document itself.
If going from the website, in the files tab, go to the folder (typically the public folder) the file is in, then at the end of the file listing (the right hand side) is an arrow that gives you a drop down list when you click on it, that you can select the "copy public link" option.
Then all you need to do is paste in that link in some URL/BBC tags, for direct access to the file.
Hope that helps