If you need to attach a file of any description to your invoice, then the best way to do this is to use the carbon copy email. The carbon copy email is an exact copy of the email that your client receives, containing your invoice PDF attachment. It is always simultaneously sent to your own email address, whenever you send an invoice to someone through the app. Please note: this carbon copy email will always go to the email that you used to register your Albert account with. If you however do not wish to first send it to your client, then the simplest thing to do is to send the invoice to yourself from within the app. Please note: by doing this, you will receive two copies in your email inbox, the original and the carbon copy, so you can use either email, as they are both exactly the same.
So simply forward either one of these emails onto your client(s), whilst attaching any necessary files in the process.