Invoice number that increments each time a PDF if opened

@Jeff Bratton I see you haven't had much response to your question. I know what you are looking for and while there might be a way to set it up, are you sure this is a good idea? Honestly, I am not trying to talk you out of finding a way to do what you want to do! If you are creating invoices that need to have a unique number assigned each time, there are accounting programs which can be expensive and difficult to learn. So perhaps look at some free (cloud based) invoicing apps. They only charge transaction fees if you use their app to collect credit card payments on your invoices. You can customize invoices with your own logos and colors and write anything you want on them. You can duplicate the invoices with a click and it will automatically assign a new invoice number. Then you can change the name of the client or whatever else you need. When done, you can save your invoices as PDF files. Some apps have no charges. You might check Wave Apps Invoicing as an example. Please note that I have not used this app myself and I have no affiliation with the company. You can do an online search for "Invoice software that's easy and free to use." I hope that's helpful!

This is not a very good idea. You're going to get plenty of duplicates if you do it... Use random numbers, or assign the invoice numbers in a centralized location, instead.