There are several reasons that could cause this issue. Please try to check the following steps to see if you encountered this problem.
The App doesn't Support the File Format
PDF Reader Desktop versions (Mac & Windows) only support viewing and editing PDF files. If you import the file that PDF Reader doesn't support, the file won't be able to be opened. You could try to turn the file into PDF format before you use PDF Reader (e.g. Image to PDF and Microsoft Office file format to PDF)
The Files are Broken
If the file has already broken before you saved it to the PDF Reader app, the file can not be opened, either. If your PDF files are accidentally locked by passwords with no reason, please do not delete the file and send it to firstname.lastname@example.org. We’d be happy to assist you.