I have downloaded over 60 files on my Galaxy S7. Each and every single one of them read fine on my phone when downloaded. I read and enjoyed several of them.
I then uploaded them to the cloud so I could access them from my computer. When they uploaded to the cloud I renamed from the random number names to the names of the books and manuals that the books actually had. Never could figure out how to do that from mobile and that's not the issue at hand here.
When I go to pull one from the cloud onto my phone now I get an error message saying "There is no app that can open this file".
Did renaming them reformat the file to a type my phone can no longer open? Or corrupt them somehow? They read fine from my computer still. Keep in mind each and every single one of the files worked fine on my phone prior to this.
In addition to solution provided by Bob, I was also curious what app and version thereof created these PDF's and how and where you uploaded them the cloud? Adobe has a couple different cloud hosting services, Creative Cloud file storage and Document Cloud. If this behavior is something which can be recreated we can report it to our developers to get fixed.