How is Acrobat DC document encryption and protection so easily cracked?

We would like to protect certain PDF documents from copying, modifying, printing, converting to Word using encryption and a permissions password.

I have successfully tested this functionality to my satisfaction using Acrobat DC to try and modify test documents.  However, if I try free online tools to convert PDF to Word, they seem to be able to crack the protection with little or no problem - and example is [ Admin: removed link ] which converts an encrypted file protected with a 40 character complex password in seconds.  Others ask for password or get stuck during the process (as if they are attempting to brute force the password) but smallpdf.com breezes such files.

So, what is the point of this Acrobat functionality if it is so easily circumvented?

I would be grateful of any advice on how to properly protect my documents.