Having the same problem/message warning when uploded and viewed the PDF with build-in pdf.js viewer. PDF is showing up as a blank white pages while selecting some text, the content can be seen in the viewer.
Tested with
Firefox 90.0.2 (64-bit) - NOK
Chrome Version 92.0.4515.107 (Official Build) (64-bit) - NOK
Microsoft Edge Version 92.0.902.55 - NOK