As regards the font encoding. If you look at the PDFs you made, the encoding is different in both the AI and XDP versions. While that may nail down the difference between those PDFs as regards GMail's Preview, it will not be consistent in Apple Preview.

There are tons of threads about Apple Preview's inability to consistently display PDFs made from both InDesign and AI on the Adobe forums. The advice from Adobe directly and all the regulars on both forums is to tell a client to not use Apple Preview.

Take care, Mike