Microsoft has released an optional out-of-band update for all supported Windows 10 versions to address an issue preventing customers from opening PDF documents using some applications. The KB5004760 emergency update is available for devices running client editions of Windows 10. Internet Explorer users are only affected by this bug if they toggle on “Enable 64-bit Processes for Enhanced Protected Mode”” is enabled in the Advanced tab in Internet Options. A PDF might render as just a gray background when using the Adobe Acrobat plug-in.”
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10-kb5004760-emergency-update-fixes-pdf-opening-issue/