Every name of the document in odoo (being generated as a report) comes from the Sequences and Identifiers. Now in order to find this sequence (name which is predefined only the number is changed) just navigate to Settings --> Technical --> Sequences & Identifiers --> Sequences. Make sure that the developer mode is enabled in order to find the Technical menu in the Settings. After getting here you can search in the search bar for Invoice, Quotations, Delivery Notes, etc. 

Now click on customer invoices and change the starting name or the PREFIX as needed. If you are working with projects mostly you can place in PREFIX the name of the project or anything else that you would need. I hope this answers the question. Please upvote if it does.

Riste Kabranov,

Python/Odoo Developer @ NEBIZ IT DOOEL