What encryption is used
When you add a password, the PDF is encrypted with AES-256 (PDF 2.0, security revision 6). It works in current versions of Adobe Acrobat, Chrome, Edge, Firefox, Preview on macOS and most mobile viewers. Very old readers from before 2006 may not open it.
The strength of the lock depends mostly on your password: a long, unique one is far harder to guess than a short one.
Choosing and sharing the password
Longer is better: a passphrase of several words beats a short mix of symbols. The strength meter is a quick guide, not a guarantee. Don't reuse a password from another account.
Send the password through a different channel than the file. For example, email the PDF and text the password. The password can't be recovered, so keep a copy somewhere safe.
Why it's safer than upload sites
Most online unlockers send your PDF, and often its password, to a server. A bank statement or ID card is exactly the kind of file you don't want sitting on someone else's machine.
Here, the file is read and processed inside this browser tab. The page's security policy blocks network requests to other sites, so the file and password have nowhere to go. After the first visit it also works offline.
Questions
Which encryption does it use?
AES-256, the strongest standard PDF encryption (PDF 2.0, security revision 6). Very old PDF readers from before 2006 may not be able to open it.
Can I get the password back if I forget it?
No. There is no reset and nothing is stored. Keep your original unprotected file until you're sure the password is saved.
Do the print and copy options really stop people?
They're flags that well-behaved viewers respect, but some tools ignore them. The password required to open the file is the real protection.
Is my file uploaded to be encrypted?
No. The file is read and encrypted in your browser and never sent anywhere.