Keep the receipt. When you eventually play the piece with an orchestra, you will need to rent the full orchestral parts from Presser (which requires a proof of purchase for the solo part). Having your official PDF receipt saves you time later.
Under international copyright law (Berne Convention), copyrighted works are protected for the life of the author plus 70 years. Consequently, Liebermann’s concerto will not enter the until the 2060s. liebermann concerto flute pdf link
Visit Sheet Music Plus and search for "Lowell Liebermann Concerto for Flute and Orchestra Op. 39" . Select the "Digital Sheet Music (PDF) – Flute and Piano" option. Pay the ~$20. The file will hit your email inbox instantly. Keep the receipt