You can rename the PDF Headers/Document's title located at the upper right of PDF. e.g. Bill, Quote, Service Order etc. It also changes the respective display name across the entire application.
Manage all your Company's Info/Details and set currency so that those details will appear on document's PDF (Invoice, Estimate, Purchase Order, Credit Note)
Change/Set document's ID/Number (Invoice, Estimate, Purchase Order, Credit Note) as per your choice. You can customize your document's number and use a combination of numbers, letters, and symbols.