[supernova87a]

There's no surprise. Gmail does.

If you search for a text string in Gmail, it will return emails that contain that text only in scanned images or PDFs that are in your mailbox.

[odensc]

That doesn't mean they're crawling the URLs, just that they're indexing the content of PDFs/images for your search. Which is, honestly, a pretty useful feature. Whereas Facebook is doing this without providing any value to the user.

[_8j50]

I've worked with email security appliances that will visit the pdf url when detonating the attachment in a Sandbox. Dynamic analysis helps find 0day's ,sometimes the link leads to something worse,the link many times is a url shortner too