Boehm writes long, singing lines over arpeggiated piano chords. Do not rush. Use heavy vibrato on the sustained high notes (High B and C). The challenge here is breath control—plan your breaths at the ends of the 4-bar phrases.
Amateurs often make the Polonaise sound like a waltz. The rhythmic figure (dotted eighth note, sixteenth note, then two eighths) must feel noble.
The safest and most legal source for the Böhm Grand Polonaise flute PDF is the International Music Score Library Project (IMSLP). Search for: "Böhm – Grand Polonaise in D major, Op. 16". You’ll typically find:
Note: Beware of poor-quality scans or unauthorized arrangements on less reputable sites. Always check for missing pages or illegible markings.
IMSLP is the Wikipedia of public domain sheet music. Search for "Boehm, Theobald" then look for "Grande Polonaise, Op. posth." or similar titles.