I am using Acrobat XI Pro => Require a password to open document => use a password that is 'Best' with the 4 gray rectangles on the right lighting up green => Compatibility: Acrobat X and later, Encryption Level: 256-AES
In other words, the above is the strongest security that Acrobat XI Pro has to offer by way of encryption. But just how secure is this strongest security? Any of the experts here care to give a lay person a feel for the level of security in case the encrypted pdf file gets in the wrong hands?
It would be very hard (take a very long average amount of time) to crack the security with that password and encryption level, but both the "document open" and "change security" passwords should have the same high level of strength since both can be used to open the document. Be sure to use passwords that aren't susceptible to dictionary attacks.
Check out this site: https://www.grc.com/haystack.htm
but read the background information so you know what it's showing.