This French startup claims it is able to run some Javascript in the PDF, therefore notify you when a customer reads your offer, tell you which part he read, where he stayed the longest, etc. Does PDF support JS?
You can run JS in a PDF. You can even embed Flash in a PDF. We tend to think as PDFs as an innocuous document format, but there's a lot more than that baked in.
Adobe actually offers "features" like readership tracking in PDFs as part of their commercial offerings.
Thankfully about half the "features" of PDF like js tracking and restrictions on editing or printing are ignored by almost every PDF reader not made by adobe.
https://www.tilkee.com/