The cost of uploading a PDF of links is probably not much less than the cost of following those links on your own. So I don't think you gain much by leveraging Facebook in this case.

What if I create a PDF with this content...

https://news.ycombinator.com/item?id=1

https://news.ycombinator.com/item?id=2

https://news.ycombinator.com/item?id=3

and so on, until 10,000,000? Perhaps Facebook starts opening every link using 10,000 parallel threads. Can you really replicate that from your connection at home? Perhaps even the sysadmin of your victim site has whitelisted all Facebook IP addresses so their crawlers get a free ride.

There's a fair chance the service which generates these outbound requests throttles itself, both with respect to how many requests it makes against any one domain/IP, or how many errors it will provoke per time period, or before human review.

Maybe, maybe not

I would imagine it queries sequentially so it wouldn't have too many parallel threads. Not much reason to parallelize.

It's this kind of thinking that unearths the bugs

I'm sure you could just point all those links to a domain you own (on some poor, unsuspecting VPS) and see what happens? Might be playing with fire, though.

I remember something like this was possible back in the day with Google Sheets. You could embed a URL as if it was an image in each cell of a sheet and it would make thousands of requests. I don't remember the details.