If Receipt Management is enabled in your instance of Luminate Online, normally these receipts can be edited under Setup > Receipt Management. See this article for more information.

However, if no receipts can be found in this location, check the following configuration.
  1. Select Setup > Receipt Manager
  2. Select the Global Options tab
  3. Under PDF Receipt Options, see if the checkbox is marked for Attach PDF Receipts to Autoresponder
  4. Under PDF Receipt Options, see if the dropdown menu for PDF Receipt Provider is set to Default
If so, the .pdf receipt being generated is the one built into Luminate Online as the default template. This template is hard coded into the system and is inaccessible by either an Admin or Support staff. The recommendation is to create and upload a new .pdf receipt template. When configuring the template, you can select the applications for which the receipt is used.