Will every PhD student from a reputed institute land a postdoc position?
There is a filter, surely.

Actually, at least in German system there are two filters: PhD → postdoc and postdoc → tenure/tenure-track.

But you are missing out one issue. Even if all people doing postdoc probably would like to get a tenure-track position (but do not), not all PhD students would like to get a postdoc. Heck, not all PhD students even finish their PhD! But the major point is: not all PhD holders want and do stay in academia.



Ok, you do want to stay in academia. There are three major issues:


Publications
Teaching experience
Funding acquisition.


The detailed answer is probably too long and too country-dependent, but typically you'd need these three major issues to be covered, if you want to land a tenure-track position.