[Arlis]

You give each Bitcoin its own password which you only know and keep elsewhere.

Yes, super secure. Even if they get the paper, they can’t get your Bitcoin.

AND, unlike dollar bills, you can have as many copies of your paper Bitcoin as you want.......

[Errant]

You give each Bitcoin its own password which you only know and keep elsewhere.

That's an idea, or you could alter say the first 10 numbers of your hard copy by maybe 10 lower on the ACII table. Lots of ways to make it practically impossible to decipher.