The best passwords these days are passphrases with mixed case and non-alphanumeric characters.
I personally use 24-32 character passphrases, with combination of character sets. A passphrase is easy to remember, it’s a sentence you modify into a password.
However, document passwords often are pointless and are not secure at all, the implementation of document passwords makes it very easy to crack the password regardless of length, you are better off encrypting the document with something like PGP.
Here is the reason for passphrases, note that even 34,000 years is not considered secure enough, this halves every 18 months and you can just “rent” thousands of computers on Amazon or the Dark Web. You need something that take millions of years to crack.