pdf security permission valid only for windows

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 @qubit! 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).