Open the Google Doc
Click Share in the top right
Under General access, set access to Restricted
Remove anyone who should not have access
Click the gear icon in the Share window
Uncheck Editors can change permissions and share
Uncheck Viewers and commenters can see the option to download, print, and copy
To prevent editing, change the person’s role to Viewer or Commenter
If needed, make a copy and share only the copy
To lock specific content, use File > Version history > Name current version
To protect a section, place it in a separate document with restricted access