Although I know little about Serbia, and their PPL theory requirements...
In general terms for countries that I do know, France, England, and a few others... Require very little other than self studying, with the books and question banks in order to pass your theory exams. There are schools that offer specific "ground school instruction for PPL" but this is not required (to the best of my knowledge & understanding) in order to sit the PPL exams.
There are however, theory requirements both in terms of content, and hours, for sitting any of the CPL / IR / ATPL exams, whether by distance learning or by full time learning at an ATO.
Even if you were required to sit in a room with an instructor for a certain number of hours prior to sitting your PPL exams, I would recommend reading the books anyway, as it will give you a head start!
I hope this helps.