Securing access to invoice PDFs

What version of Invoice Ninja are you running? ie v4.5 / v5
v5.3.9

What environment are you running?
Docker

Have you searched existing issues/requests?
Yep

I noticed that in the client portal and the invoice-only view (with the full client portal disabled), the PDF filename as it is stored numerically is visible in the webpage source code. This means that any client can seemingly just take the URL to their invoice PDF and just tweak the invoice number in the URL to see other people's invoices.

Am I missing an option to secure this?

Thanks.