Remove encryption settings
You can remove security settings from a PDF file that is already opened. However, it requires the correct permissions password if the PDF file has already been encrypted with permissions password security.
Remove password security
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Open the PDF and do any of the following:
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Choose Security > Encryption > Remove Security Settings. Click OK.
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Open the Document Properties dialog box, click the Security tab, and choose No Security from the Security Method drop-down menu.
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Your choices vary according to the type of security applied to the document:
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For documents protected by open passwords, click OK.
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For documents protected by permissions passwords, in Security tab, choose No Security from the Security Method drop-down menu.
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Click Save Now to apply the changes to the current document, and click OK.

In order to remove permissions password, choose No Security, and enter the correct password.
Remove certificate security
Open the PDF and do any of the following:
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Choose Security > Encryption > Remove Security Settings. Click OK.
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Open the Document Properties dialog box, click the Security tab, and choose No Security from the Security Method drop-down menu.
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Click Save Now to apply the changes to the document. Click OK.