Howto link to a file of a different dataset

[TroubleAllWays]

In list blablA, there is a fileupload element to a pdf file:
ID1 blabla1 bla1.pdf
ID2 blabla2 bla2.pdf
ID3 blabla3 bla3.pdf
...

Now I have created a second list blabaB, in which there is a datajoin to list blabla:

ID1 blob1 blablA___ID1

How can I add in list blablaB a link to the specific pdf file of dataset blablA without entering it in every dataset? I have the feeling, the calc element would help, but how?

I just want to have a link in form or list of blabalB, which opens the corresponding pdf file.

Any idea would be helpful...

Thanks a lot