It would be a good idea to save the PDF Invoice files to a secure folder on the server, a folder you can set the path to in config, preferred outside web root on servers supporting this.

Sometimes you need to download and print the invoice again, but if any info is changed in between (like company info/address), the new print/save will be different than the one the customer got - and you might get in trouble with the tax authority. Invoices shall not be possible to change/manipulate after generation in quite a few countries.