There could be various reasons that cause this issue. Please check the following reasons to see if they can solve the current situation.
The App doesn't Support the File Format
PDF Reader Desktop version (Mac & Windows) only supports viewing and editing PDF files. If you import a file that is not supported by PDF Reader, it won't be able to be opened. To solve this situation, please try to convert the file into PDF format before you import it into PDF Reader (e.g. convert images or Microsoft Office files to PDF).
The Files are Corrupted
If the file was already corrupted before being imported into the PDF Reader app, it won't be able to be opened. Similarly, if your PDF files are unexpectedly locked by passwords, it might potentially be the same reason. If you encounter the situation above, please do not delete the file and send it to firstname.lastname@example.org. Our support team will be happy to assist.