Name the invoice and receipt files with the invoice number
When downloading the invoice and the receipt after payment. Name both files with the invoice number to keep them related to each other on the user's system.
Currently your system names the the invoice file with the invoice number in the filename but the receipt file is just named receipt.pdf - a generic file name that is not in an way related to the invoice it belongs to.
I agree with this. Ideally they should both download as two files with a single click and be named <vendor name> <invoice number> <date> with "receipt" appended to the name of the receipt download.