Adding unique value to the URL as query string will not change the current behavior, as it seems the root cause is somewhere else. We checked out production app and confirmed there is a tendency where the PDF size is relatively large, then it fails to open the PDF file through appsheet.com server. I m purely guessing some breaks in CDN are currently happening. Anyway, our prodution app is out of use due to this issue, we simply hope a quick fix will be brought.
I am not very confident that AppSheet support understand that this is a SYSTEM issue that has started happening this week, and NOT an issue with a particular APP.