org-link-file-path-type

org-link-file-path-type is a variable defined in `org.el'.
Its value is

adaptive

Documentation:
How the path name in file links should be stored.
Valid values are:

relative Relative to the current directory, i.e. the directory of the file
into which the link is being inserted.
absolute Absolute path, if possible with ~ for home directory.
noabbrev Absolute path, no abbreviation of home directory.
adaptive Use relative path for files in the current directory and sub-
directories of it. For other files, use an absolute path.

You can customize this variable.