.... I am fairly sure that you will find that only people with the same iOS devices and software will be able to see your files - This is true generally.
I have similar iOS devices to you and so can't test. I have iPhone 7 Plus, not 8+, and iPad 5, but all running 12.1.
No, it’s just a link to a raw file. As long as the OS supports it anyone can view it.
My GF can see it on her iPhone 5S fine, as can my son on his 6S. Yes, we are all running IOS12.1, which supports the .CR3 format.
Now here’s the interesting bit.
IOS12.1 will import these files in to the the photo app. All good. Thumbnails are generated as with any other files. The interesting bit is until you have imported one as above, Dropbox can’t work with them.
I tired this on an old iPhone 5S that had IOS11.4 on it. The phone and DB didn’t want to know, as expected. Updated to 12.1 and Dropbox still didn’t want to display the files I had uploaded. I connected the SD card reader and imported one .CR3 file. I then opened DB again and it would now preview the files.
I tried this on my sons 6S as that has never had the SD card reader before and I got the same result before and after.
So yeah, any device running IOS12.1 will work, but for now you can’t share the file to another device via DB unless that device has, or has had a .CR3 file in the photos app.
That’s my findings anyway.