First, I would presume that you are protecting this in some way so only the recipient of the invoice will be given authority to access it. One way to do this is to simply provide a link to the .pdf file after the user logs in. A better way to do this, though, would be to convert the PDF file to a web page (so it can be better protected), using one of the utilities previously mentioned.
You would need a script of some kind to do that. I imagine you are not on a high budget, so getting a custom build script is out of mind. But why not look at all ready made scripts at HotScripts (http://www.hotscripts.com).