Hello Jan,
The way Devonthink handles this is by the creation of a separate document which directly links either to the page in a pdf file or to a search of the specified text.
The easiest way to do this is to use the built in Annotation Template 'Data->New From template->Annotation (Cmd Opt Shift A). The Annotation Template creates a new rtf document to insert notes and tags that are relevant to the annotation.
There are a number of user contributed scripts on this forum which add additional functionality to the basic idea of the annotation template by for example collecting all the annotations for a single source document into one rtf documentor by collecting all the annotations for a single source document into a group.
This approach in fact proves far more flexible that adding text directly to pdfs.
Frederiko