The Report was found in the Hoover Institution Archives in Stanford among the papers from the Ministry of Information and Documentation of the Polish government-in-exile by American historian David Engel. Engel noticed, without commenting, that pages 6,8,9,10,11 of the report had two versions; the second version - which had a gentler account of the Polish attitude towards Jews – was edited by Karski, or at least acknowledged by him. This is confirmed by his handwritten note on the original. The milder version concerned the Nazi occupation.
Karski’s report – read in David Engel’s article – here