How do I password-protect a PDF document that doesn't let me change the security settings?

[LauraMichelleSP]

I want to password-protect my PDF document (so when I email it, it can only be opened/read by intended recipients as it contains confidential/sensitive information) but when I go into document security, I'm unable to change the Security Method (which is currently set to 'No Security' - not by me).  There's a 'show details' icon but it's grey so I can't click on it.  What can I do to protect this document from being opened by non-intended recipients?

[Test Screen Name · Accepted solution]

There seem several possibilities.

Are you using Adobe Reader instead of Adobe Acrobat?

Are you making a PDF/A file? (It will say so under the menu bar)

[LauraMichelleSP]

Ah, yes, I'm using Adobe Reader!  That makes sense.  Thanks so much for your reply.  At the moment, I scan Word documents into my computer and they're automatically converted to PDFs.  If I had Adobe Acrobat on my computer, and scanned in documents, would they open in Reader automatically or could I open them in Acrobat and therefore add password protection?