Have you been asked to provide a particular document not only in PDF format, but specifically in PDF/A? You are in luck as you can easily create PDF/A with LibreOffice.
As you probably know, PDF is a standard for encoding documents that is portable between systems. Then, if you are wondering how PDF/A differs from “conventional” PDF, according to Wikipedia:
Password lock PDF files with pdftk
If you need to lock a PDF file with a password on Linux, there is an easy way to do it even if it requires a terminal.