May be this is a scan of a document.
PDFs can be secured at the time of creation, or the security can be added later. You'd have to know the history of the document to know when. Looking at the file properties within Adobe Acrobat night yield a clue.
And ... ?
How can we know why someone decided to do it, if indeed that's what they did? And no, you can't know who it was, but most likely the last person/entity to work on the file.
have you established whether there is security? As someone already said, maybe it's just a scan = no text. When text = 0, highlighting <= 0.
If "whether there is security" equates to a padlock appearing in the tray then there is a padlock in the tray. What now is the relevance of this?
I've no idea what you mean by padlock in a tray. Please check security like this.
1. Open in Reader or Acrobat
2. Use File > Properties
3. Click the Security tab
4. Look what it says for "security method".
If it says "none" the file is not secured and that isn't your problem.