The only way your property can be safe is if you keep it to yourself.
The site Giveaway Of The Day (GAOTD) is even giving away a commercial tool called Recover PDF Password for free. This tool unlocks restricted PDF documents & recovers passwords of protected PDF files.
As per comments at GAOTD, this isn't also the only tool. This comes as a rude surprise to me.
Sometimes, ignorance is not bliss.
Also see:
PDFZilla overrides PDF's copy protection
PDF Anatomy 101
A Web App Is As Secure As You Make It & the Browser It Runs On