Pages v6.3 and macOS High Sierra Supplement.
In Preview File menu : Print... : PDF : Save as PDF : Security Options...
Whether I password protect the document itself, and the Copy text selection, or omit the document password, but do password protect the Copy, in either case, the copy and paste of text is not blocked to Pages v6.3 in the same user account. Either the security feature is bogus, and not working, or behind the scenes, and without user choice, the keychain has noticed that it is the same user, and automatically unlocking the copy protection.
If you open the PDF in Adobe Acrobat Reader DC, there is no copy password challenge, but the Edit menu : Copy is disabled upon selecting text.
If the PDF is opened by Adobe Acrobat Reader DC on Windows 10, selected text will be met by disabled Edit menu : Copy, even though text selection is possible.
In past versions of Preview, one would receive a warning dialog that copy protection was on, and a password challenge — even on the same user account. This has apparently is now broken in High Sierra.