Yes, I had exactly the same symptoms and also on the same hosting provider and type of hosting.
The thing is that any browser that runs on iOS uses WebKit as its underlying code. This means that really, Chrome, Firefox, Safari, and Edge on iOS will all behave much the same way. I’m not sure if this is the case on Macs, but definitely for me it was a problem for Safari on Mac.
It took me a LOT of digging around, but I believe the root cause was to do with HTTP2 vs HTTP/1.1
I *think* Siteground is almost forcing everything to be over HTTP2 now. This should be fine, even for the browsers on iOS. But for some reason PDFs served up by Mdocs clash and seem to want to only go down HTTP/1.1. Or something super technical like that which I don’t really understand!!
Anyway, I ended up moving my site off of Siteground and on to a VPS on AWS Lightsail. This completely and instantly solved the problem and the bonus was a huge speed increase!
If you’re not keen on managing your own VPS server, I recommend looking around for other similar hosts to Siteground, and see if you can find one that will offer a 30 day free trial or something like that. Setup a vanilla wordpress and install mdocs, and see if that works.
Fingers crossed for you!
Steve