To do this, you will need a full-fledged editor Adobe Acrobat DC. Using Acrobat Reader you will not be able to remove the restriction.
Acrobat Reader only lets you see the enabled restrictions.
Open the PDF file. In the File menu go to → Properties… (or use the shortcut Ctrl+D) and open the document properties window. In the Security
tab see the enabled restrictions, click Change Settings, enter the password and disable password protection in the Password Security window.
Save the changes in the PDF file.