If you are a student or faculty member, check your university’s music library database (e.g., Naxos Music Library or Alexander Street Press). Many universities have purchased digital licenses allowing students to download verified PDFs for educational use.
No legitimate source will give you a free, full-length PDF of the entire Four Seasons for piano trio because the work is under copyright. However, you can obtain by purchasing from these trusted online retailers: piazzolla four seasons piano trio pdf verified