When creating the download PDF, add the receipt number to the start of the file name.

It isn't possible to have two files called (for example) "Syd Smith" in the same directory. Adding the tax receipt number makes the file name unique, (for example) "123-Syd Smith" and "362-Syd Smith".

Adding the receipt number to the front of the file name also makes them sortable and searchable by tax receipt number.