I suggest u check with a Japanese...
But my check earlier with a friend earlier is that のを is the correct one...
のは in this case is inappropriate... Like what Niaosan and i said... 忘れる is a 他動詞, in this case, the "object" is
"The matter about bring the documents tomorrow"
If you put は、the meaning will change. I don't really know how to explain to u... But ya... The meaning will change.
http://www.jisho.org/sentences?jap=のは&page=2&eng=
Looking at the examples in that link, u would see the number of examples used for のは with a verb at the back.
This is from Minna no Nihongo. のは do not use a verb at the back.
From this book, u can see that 「を」 is used as 動作の対象, whereas for 「は」 is used as 主語提示. if u know what i mean.
"Also は not equals to 'is'. です is more appropriate."
I beg to differ. The combination of ~は~です means is, am, are... Not just は or です。 Not sure if your Japanese teacher has told u before.