There are two kinds of security passwords. Document Open and Permissions.
Document Open is what you want and if you use a password, it is very secure. Only the person with the password can open it to see the contents.
Permissions is quite different. You can open the document, but Printing and Editing can be disabled. That is the one which can be unlocked with third party tools.
So open your PDF, go to File > Properties > Security Tab and check that first option. Set a strong password and only the people you share the password with can open it and see the contents.
I did see a reply to my comments, but it's no longer there. Try again and if the answer I gave settles your question, mark it as "correct" to let others know it's been solved.
If you open the password-protected document having other, say, 40 documents open in Acrobat Reader, you will not be prompted for a password. Or at least, that's what has just happended to me when opening my bank statement (serious thing, right?). I had opened some other statements before, as well as other non-protected documents, and kept them all open. After some 40 or 45 documents, Adobe Reader DC is not asking for a password anymore, even when the documents are password-protected. Where can I report this as a bug? Is this the right place? FYI, I'm using:
OS: Mac OSX 10.12.6
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It's best you open your concern as a new topic. Tacking a different problem on to another thread gets it little if any notice.