Would it be ok, if I place a link for a croatian PDF-manual on the croatian „Books and Manuals” web page?
Is it necessary to ask the author/university for permission to link the page?

I would add it like this:
<h3><em>Inkscape ili vektorska grafika za svakoga</em></h3>
<p>Autor: Dominik Kenđel. <a href="”>PDF verzija.</a></p>

I found this site at the University Computing Centre (SRCE) of the University of Zagreb, Croatia.
This would be much better for croatian users, than having to read the english versions. It is based on „Inkscape0.91”, but I don’t think that should be a problem.