I assume, the tracking happens before the recipient sees the message. This could be used to track sent messages, not received.

Yep there’s still no “open” tracking on PDFs, at least afaik from a pixel/beacon standpoint. Entire businesses like docusign are built with that value prop in mind.

It's to protect from malware and phishing. Plenty of other companies do this too and it is reasonable. Someone else here put it really well.. damned if you do and damned if you don't.