Have you pressed on the Set Passwords button in the Security tab of dialog File>Export As>Export as PDF?
A secondary dialog pops up with sections Set open password and Set permission password. There is a Help button which gives the answer to your question. Even without going so far, the Security tab already gives a summary of the security passwords (or the absence thereof).
As a general rule, always start with some navigation in the built-in help and ask here when you could not find what you were looking for.
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