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How do I change my password for my pdf files on adobe acrobat
Not possible with Acrobat Reader.
Adobe Acrobat Reader Mobile App and Acrobat Reader DC desktop app both are freeware. It doesn't allow you to change the document security.
To make any changes in Document properties or its security setting you would require Acrobat DC subscription. https://helpx.adobe.com/in/acrobat/using/securing-pdfs-passwords.html