The Siddur Tehillat Hashem is the authorized Chabad-Lubavitch prayer book following the Nusach Ha-Ari (Rabbi Isaac Luria) rite, as arranged by Rabbi Shneur Zalman of Liadi.
Official PDFs of specific sections or the full annotated siddur can often be found through Chabad-related organizations or digital libraries: Siddur Tehillat Hashem English - MCHIP siddur tehillat hashem with english translation pdf
If you encounter a PDF claiming to be Siddur Tehillat Hashem English: For legal, high-quality, complete text: Use the Kehot