org-agenda-text-search-extra-files

org-agenda-text-search-extra-files is a variable defined in `org.el'.
Its value is

nil

Documentation:
List of extra files to be searched by text search commands.
These files will be searched in addition to the agenda files by the
commands `org-search-view' (`C-c a s') and `org-occur-in-agenda-files'.
Note that these files will only be searched for text search commands,
not for the other agenda views like todo lists, tag searches or the weekly
agenda. This variable is intended to list notes and possibly archive files
that should also be searched by these two commands.
In fact, if the first element in the list is the symbol `agenda-archives',
then all archive files of all agenda files will be added to the search
scope.

You can customize this variable.