Reason  can't  open a downloaded PDF file on your mobile:

• The PDF document is corrupted: If a PDF document is corrupted, it cannot be opened on any computer, not only your smartphone. The document may become incompatible with the device due to a saving error or a code in the file format. It usually occurs when a file is not properly downloaded, and the easiest approach to determine whether the issue is with the file or with your cell phone is to access it on another device.
• A password or decryption tools are occasionally required to open a PDF document because it is encrypted. If you ignore it, you will get a blank window or be unable to open the file.
• PDF file formats are not supported by the app. - Apps can sometimes interfere with a phone's essential operations, preventing it from working correctly.
• Normal PDFs can be viewed immediately from your phone's download manager, but there are other options, such as Google Drive or One Drive, which are Google apps that are compatible with this sort of file.

Read More: Why won't my computer open PDF files? How can I fix it?