did she know how purab stole her diary and read it
Purab did not steal her diary. Aryan stole the entire folder which contained her diary and gave it to Purab. Purab did not want to read her diary initially, but when the wind blew the pages open, he was naturally tempted to read further. Its human nature to be curious about someone you love.
Besides, I dont see what the big deal is about him reading the diary. If they had followed Betty La Fea exactly, Armaan would have read her diary anyway. I dont remember Jassi giving him permission to read it either.
and how he stole stores plans papers
You are mistaken. Purab SAVED Gulmohur by finding out the person who had stolen their bid for the store plot - ARYAN. He did not steal anything. Jassi kept blaming him for every single thing and he just wanted to prove his innocence. That man with the spectacles sent the folder to Purab which showed that Aryan had bought the land. Watch the episodes again and you will see what really happened.
then y sld she know abt armaan and that too when he has done nothing as compared to purab ?
Yeah, Armaan did NOTHING at all. All he did was manipulate and use her, make fun of her, lie to her that he was in love with her, treat her like a doormat and try to destroy her when she became a CEO by spreading false rumours in the press.
Sanjana is absolutely right. Whatever Purab did was with good intention. He wanted to help Jassi all the time. I am not saying that what he did was always correct (like spying on Jassi through Arundhati), but his motives were pure and that of a man who wanted the best for someone he loved.
He honestly thought that Jassi needed to be saved at that point in time, but now Jassi is proving to him that she is quite capable of taking care of herself, which is why he admires her all the more.