You may want to attach an invoice to a particular transaction:

1) Head to your Transactions page

2) Scroll to the relevant transaction and click anywhere in the white space on that row. This will open up the transaction metadata

3) Scroll down the pop-up until you see the 2 Invoice options:

a) Within Add Invoice you can upload an invoice number and a file if you wish. If you have multiple invoices, merge them all into 1 PDF and upload it via 1 file

b) Within Link with an existing file, you can re-use a file you’ve previously uploaded