Hope you are doing well and sorry for the trouble. As described you are unable to open the PDF files.
What happens when you try to open the PDF file? Do you get any error message? If yes, please share the screenshot of the same for a better understanding.
Would you mind sharing the workflow/steps you are doing to open the PDFs? Are you opening the PDFs in a web browser? If yes, Web browsers use their own PDF mechanism to render and display the PDF files and they are not Adobe dependant. Please try to download the file to your computer and then try to open it with Adobe Acrobat Reader DC and check.
Let us know if you experience any trouble and need more help.