Excuse me if I missed this point in the answers.
Can I put a password on documents when saving them in LibreOffice, as you can as in MS Office?
Have a look on the ‘Save As…’ Dialog. You will find an option ‘Save with password’ there. The password will be used as an encryption key in this case. Recovery of a document having forgotten the password will be next to impossible.
https://wiki.documentfoundation.org/images/b/b0/GS42-GettingStartedLO.pdf, pp27
For ‘Calc’ documents there is also a protection against inadvertent changes and a bit more. It can be hardened by setting a password, This kind of protection is weak and easily avoided.
https://wiki.documentfoundation.org/images/4/47/CG41-CalcGuideLO.pdf, ‘Hiding and showing data’, pp69
(This is also about prohibiting changes, Some paths through the menu tree have slighty changed since the guide was published.)