By the way, even if a PDF file is opened within the browser, it must be downloaded to the local machine itself into a temp folder somewhere before that happens. That's why you can then save it elsewhere if you so choose. But things that get thrown into these locations are constantly being deleted as other stuff needs the space.
Even the file that opens for you in Adobe Reader DC can be saved out of Adobe Reader DC if you want a copy.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong.
~ H.L. Mencken, AKA The Sage of Baltimore