Hi!
When I set the permission password to export to pdf files (Files → Export to pdf… “Security tab”) the security settings seem to be valid only for Windows machines, not for other GNU/Linux systems like mine. I set everything to ‘not allowed’ (printing, copying, etc.) but I can still do all these things on my machine and other Linux machines, only not on Windows… why is this? I’m running LO 3.5.1.2
Looks like you have found a bug. Other one was already filed to Bugzilla that is loosely connected to this one in that PDF permissions are also not correctly set, although what you found seems more serious. Instructions on how to file a bug report are located here.
It’s also possible that the PDF reader OP is using on GNU/Linux doesn’t support or disregards these so-called “security” features.
Excellent point @qubit1! Never thought this could be the case, that a well-known often default pdf reader (although I don’t know exactly which one OP used) is designed to bypass security permissions, but after testing it with documents created with other apps eg. pdftk it is obvious that [continued]
it’s not LibreOffice’s fault. Sorry for false alarm. It seems aforementioned pdf reader only takes into account user password (for opening document) but disregards all permissions. This raises important question: what’s the point of setting pdf permissions when they can be trivially bypassed?
@luyu – You’d have to ask the people who wrote the spec that! 
As people have continually (re)discovered, with perhaps the latest (re)incarnation being SnapChat, you basically have two options when it comes to security of an item: You share it (unencrypted) or you don’t (encrypted).