JPGs are also a lot safer as PDFs can ping remote resources using carefully hidden beacon images.

Although that said, I sometimes use this to see who opened my files. I once left hundreds of these on a very popular cloud hosting provider (not naming names), and somebody working there was stupid to open the PDF on a machine connected to the internet, thereby proving abuse by employees and proving any random stranger can access 'your' files in the so called 'cloud'.

Look up 'honeydocs'. Some interesting articles about this technique