Is the PDF file unique to that one user? If so, save it with their username in the name of the PDF.
So, the invoice08-18-2014.pdf would become erniealex-invoice08-18-2014 or something like that.
Then, when they log in, display a list of all of the invoices that start with their username which they
can select to view/print.
If it is not unique to them, you would have to keep a list of the ones they are allowed to view and show that list to them so they can select them.
To upload a file to a server is easy to do with PHP on a website. But, you need some way to only show them to the correct user’s. So, as Topcoder mentioned, store them into different folders with the usernames or user-id’s for the folder name, or encode the name itself to show the user that can view it.
If you figure this out let us know. If not, give us more information on what you need to do. We can help!