PDFs are notoriously hard to produce and to enable the invoices to be edited by Associations and still saved as a PDF we would need to go through a thirdparty service. We currently do this for capitation reports.
Each PDF would cost US$0.04 regardless of length.
Billing would either have to be absorbed by the Federation or a tracking system would need to be built in to record each PDF request and allow the cost to be passed onto the Association.
An alternative is to allow the invoice to be viewed on the screen and allow the user to print or save it from there - some browsers allow the header and footer (usually the url and date) to be ommitted. This opens up some "training issues" though.