Looking for pgn openings file I am looking for a file containing several (>50 let's say) popular openings in PGN format.

The aim is to use it with cutechess to do engine testing on different opening scenarios.
I would recommend that you take a look at the Silver Openings Suite by Albert Silver. It's a hand-crafted set of 50 opening positions designed for balanced play, based on some non-trivial statistical analysis using ChessBase.

Cheers!