Hello there, @VdPSAR.
Let me share the third-party app store link so you can find tools that can help you generate a protected file. Since the password-protect PDF generated in QuickBooks is not available, you can use a third-party app.
You can visit this link for the App Store: App Store.
Also, you can go to this forum for Intuit developers. You can ask for guidance about third-party hosting applications: Intuit Developer.
Finally, you can read this article about personalizing and adding specific information to your sales forms in QuickBooks Online: Customise invoices, quotes, and sales receipts in QuickBooks Online.
Let me know if you have questions about customizing QuickBooks-generated forms. I'm always here to help. Have a great day.