First, let's go over the most likely reasons why the PDF file won't open in the WhatsApp app. Detailed explanations can be found here.
The PDF document is corrupted - corrupted PDF documents cannot be opened. Until a corrupted PDF file is restored, it will not open in any application. A mix-up or a specific code in the file format causes the device to become incompatible. It happens when a file isn't downloaded correctly. Alternatively, downloading is interrupted owing to an unforeseeable incident such as a lost or unstable internet connection.
The PDF document is encrypted; to open it, you must either decode it or input a password. If you disregard it or don't encode the data, it will be lost. or won’t open or it will throw an error saying “file could not be accessed”.
The Android phone does not accept certain file types — Applications can't always override a phone's core functionality and limitations. As a result, no matter what, users will be unable to open PDF files on Whatsapp
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