You may find "Samizdat" (self-published/draft) versions or officially released editions.
The search terms refer to (Андердог), a popular book series by Alexey Osadchuk , and its availability on Flibusta (Флибуста), a well-known independent digital library. The "Underdog" Series by Alexey Osadchuk and its availability on Flibusta (Флибуста)
Flibusta also hosts a separate book titled Underdog by an author named Grey , which is a steampunk/dieselpunk story about werewolves and science experiments, distinct from Osadchuk's series. and pdf .
The Flibusta library provides various versions of these books, though status can change due to licensing: Typically available in fb2, epub, mobi, and pdf . and its availability on Flibusta (Флибуста)