I'm here to assist you regarding invoices, m_benzcitaro.
QuickBooks Online uses Adobe Reader/Acrobat in printing, saving, or downloading documents including invoices. Let's update or repair it to fix the incorrect invoice number showing in the PDF file.
If what you're referring to is the file name of the PDF file you downloaded, you'll want it before saving the file. You can also check your drive where you saved it and change it from there.
These are a few articles that will give you more details on how to handle customer transactions:
Post again here if you need anything else. Wishing you all the best!