Do not pay for a "download link" from a random website. Yamaha Music Media does not officially sell a standalone PDF of this entire collection (due to complex international copyrights on the arrangements). The only official digital versions exist via subscription services like Sheet Music Direct (sold piece by piece).
Purpose
While there isn't a single "official" essay of that exact title, the collection itself serves as a curated journey through the evolution of Western music. Here is an exploration of why these 50 pieces are considered definitive masterpieces. The Educational Architecture 50 Classical Music Masterpieces Yamaha Pdf