Debian actually goes out of their way to patch those checks out in their PDF-related packages as part of their stance against DRM, like this example with "pdftk":

https://sources.debian.org/patches/pdftk/2.02-4/drm_fix/

This is not entirely true, you can encrypt PDFs [1] since v1.3 of the spec but the cypher is often so weak (RC4 until v1.6) they can be bruteforced in reasonable amounts of time.

[1] https://www.pdflib.com/pdf-knowledge-base/pdf-password-secur...

You can encrypt them to completely prevent them from being opened. But cgb223 wasn't talking about that, cgb223 was talking about the ability to open them but not copy text, or not print.

You can make the text uncopyable by using non-standard font indexing. The reader will be able to copy the text but it will be gobbly-gook. It forces the user to OCR the PDF or reverse the font mapping.

And ain't that a treat when clients send us PDFs to sort, print, and mail, but the address extraction fails completely.

You can also circumvent copy protection on PDFs by taking a screenshot, or taking a photo of the screen with your phone.

My somewhat less favorite pdf fact is that if you do that, you are still breaking protection, legally speaking.

Seems to be a reasonable analogy with trespass, where you are violating the law when you cross an invisible line. The need for marking the line varies considerably.

And even places with strong roaming rights tend place limits on well marked land.

So what if you open it in a postscript viewer instead of a PDF viewer? Because they are compatible formats except for some edge cases like security flags.

Postscript and PDF are definitely not compatible formats. The drawing model is similar, but the structure and code are completely different.