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I opened a PDF file in Adobe Acrobat 8.1 but this message popped up. The PDF file has no password. This is the first time I'm opening the PDF file using Acrobat 8.1.
That's very old. Can you open it in current Acrobat?
By the way, one cause of this is disk corruption, and another is ransonware attack.
Oh, but careful. Installing Acrobat DC will uninstall Acrobat 8, and you will NEVER BE ABLE TO GET IT BACK.