no document exists showing that Hitler ordered the Holocaust.

Historians, including Ian Kershaw, Raul Hilberg, and Martin Broszat, indicate that no document exists showing that Hitler ordered the Holocaust. 

http://web.archive.org/web/20191130030348/https://en.m.wikipedia.org/wiki/Criticism_of_Holocaust_denial

Additionally, Holocaust deniers claim there was never any direct statements made about executing Jews. Again, there is a sliver of truth in this statement, because there was never and outright proclamation about the extermination of Jews. 

http://web.archive.org/web/20201202184023/https://u.osu.edu/vanzandt/2018/03/08/holocaust-denial/