If you download the pdf document from the internet, save it to the device and try to show it inside PdfViewer, it will not be loaded on Xiaomi phones with MIUI 12.1. and above.
Users report similar issues with downloaded files on MIUI devices, here are a few discussions on the matter:
https://www.reddit.com/r/Xiaomi/comments/l3x5db/miui_12130_broke_file_permissions_on_mi10_lite/
https://www.reddit.com/r/Xiaomi/comments/mx05ur/file_downloading_issues_on_my_xiaomi/
It seems that's the same reason for the PdfViewer not showing downloaded pdf files as it properly shows pdf documents added as embedded resource to the solution.