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.
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You can download a pgn of ECO openings from ChessVisor. It contains 2014 variations of different openings.
The answers to the question Is it possible to download a big txt file containing many openings may also be helpful.
There is a suite of 2400 test positions in PGN format available from the gaviota chess engine website. (look for "gaviota-starters.pgn.zip")