Thanks for the suggestion. It was toggled off and now I switched it on. But after the restart nothing changed. The files that I am refering to in the text are of the type
This and that paper [[name_of_the_paper.pdf]] contains this and that info.
In fact the name_of_the_paper.pdf is suggested by Obsidian via auto completion. So it seems to know about the file? It just does not crate a link in the graph view.
Also in the preview pane I have a wonderful preview on the papers (cool feature by the way;-)).
It seems to be really about what ends up in the graph. I tried to check out the .obsidian folder but did not undertand what info goes into the graph.json and worksapce.