Quoting marco corvi <marco.corvi@elsagdatamat.com>:

kml can have references (= links) to external resources, e.g., images, through a Placemark. probably pdf too.

is it possible to define this in therion files, for example with a point of type link (uri as value) ?

There is no such symbol yet. Adding links should be also possible in PDF (at least acrobat reader supports it). I will add this feature into TODO list. Regards, S.