Preview's "Markup" not working with other PDF viewers?

[macstatic]

I often use Preview's "markup" feature to remove personal information in PDF documents by overlaying the areas I want hidden with black rectangles, like this, then save the document:

I recently sent someone such a PDF file, and in my webmail's "sent" folder clicked on the PDF attachement only to be shocked by the fact that all of those black rectangles were gone, leaving the entire contents visible!
This was is the webmail's preview feature in the browser. So I downloaded the same document on to my Mac desktop, double-clicked it to view it with Preview, and the black rectangles were back!
I can only conclude that the "markup" feature is only good with Preview.

I read some comments somewhere about this being a bug -if so, is it something that has affected several MacOS versions?
The PDF I discovered with this problem was done in MacOS 10.15 Catalina, but I've edited and shared lots of PDFs like this with people within MacOS 10.14 Mojave, so I'm worried a lot of private information has been shared unwillingly.

So how do I safely edit a PDF document in the same way, but be sure it stays that way regardless of OS platform (Mac, Windows, Linux etc.) and PDF viewer?

 

[sahnert]

recent versions of preview have a redact tool that is designed for this job. According to the documentation the redactions become permanent on closing the document.

“Select text to permanently remove it from view. You can change the redaction as you edit, but once you close the document, the redaction becomes permanent.”

 

[chown33]

Overlaying text in PDFs with black boxes DOES NOT redact the text. All it does is draw something else on top of the text. The text is still there, as you've discovered. Anyone with a moderate amount of knowledge of the PDF format can recover the underlying textual data.

It wouldn't surprise me at all to learn that a few judicious prompts to an AI would give a step-by-step guide on how to do it, and exactly which tools to use.

The same goes for SVG images, and a host of other scalable or vector formats.