Hey Howard,
We only support downloading/accessing the PDF of an invoice after it's been created and finalized. This means it's not possible today to preview the upcoming invoice in advance. This is not something I've seen asked before as usually the PDF represents the real amount they are going to pay you, which could change by the time it's created at the start of the next period.
What you're describing though is less about getting a PDF of the upcoming invoice temporarily, and more about the ability to have them pay proactively for their next period right? Something like a yearly subscription that renews in January but you want someone to be able to pre-pay in November when you warn them about the upcoming renewal. If so, this is not something we support yet, but it's something we are considering building. No firm timeline just yet unfortunately but I'll mention this to the engineering team who is looking into this feature and they could reach out when we're ready for beta testers in the future.
I don't have a really good alternative to offer today unfortunately and I'd recommend relying on the Upcoming Invoice API [1] to preview the amount and send your own preview. But you won't be able to charge in advance, not unless you change the billing cycle (unlikely to be what you want) or take a one-time payment and then add the amount to the customer's balance to discount their invoice whenever it's issued in the future (which can hurt revenue recognition).
I hope this helps though I know it's not the answer you were looking for.
Best,
Remi