When selling an ebook, it would be great to add a custom bit of text to every page footer showing the customer's name & perhaps email or other customizations so that the PDF is less likely to be shared with others.

This is a service offered, PDF Stamping, by SendOwl.

There is a Woo plugin that can do this as well. WooCommerce PDF Watermark Plugin

I'd love this level of security to discourage pirates of PDFs to others.