Why Australia declared second Test when Warner scored 335* if they allow him to play another 5-6 overs he could reach 400 right ??

Yes, he could have. The reason for declaring is to save time. You don't want to use up too much of the time you have over the five days, because if there is no result, it goes as a draw. Weather was a factor, just as it was in New Zealand (more time was lost to rain in that Test), so Captain Paine did not want to hurt the team's chances for a win in a chase for individual accomplishments.

But to think that Warner would get another 65 runs in just 5-6 overs is ridiculous. He got his 335 in 418 balls. At that rate, he would face another 84 balls to get to 400. That's 14 overs if his partner faced not a single ball in those 14, an impossibility. More like 25 overs, and if Australia had done that, and the weather had been just a little bit worse than it turned out, that could have given Pakistan a little extra life in their follow-on, stay alive long enough to force a no-result on the fifth day. Captain Paine took everything into consideration and made the smart decision to declare when he did.

They declared at exactly the right time. I have seen so many big innings where they just bat and bat and bat for records and guess what almost always happens after a batsman gets 300+? A draw.
They are playing the game at the highest level, they have to play as a team not an individual.
Sure it may seem like they had plenty of time left for Warner to go for 400 or even 500 but rain was forecast for the next couple of days, they would have looked silly and copped heavy criticism after making that many runs and risk not winning the match.
Lara's 400 did not win the match, actually it let the other team off the hook because it did not leave enough time to take 20 wickets.
Neither did Lara's 375 or his 501 or 277, none of those innings resulted in his team winning the match, they were a celebration for one man only, not much for the other 10 players.

400 or even 500 is everbodys delight however only not due to paucity of time. Sometimes luck showers you get big time for big scores and that is even better. If you allow him 25-26 overs he could have reached golden or even diamond milestone of 500.

The team always comes first.

the weather said possible rain.   And to have best chance of bowling pak out twice

Australia needs to allow time to get all the opposing team out otherwise the game will be a draw. It isn't that much of an achievement if Warner scores 400 only for his team to draw the game.

There are some game plans in every captains mind dear.

yes he could reach 400 right  now .but  I think  Total run  does not  matter . in the time. save time  and he will take  quick wicket and win the match.

because they want to win the game.

they don't depend on warner...

they play as a team...

my personal statement is yes...

Yes, he could have. The reason for declaring is to save time.

5-6  overs isnt sufficiant...

he could need 10-15 overs more for 400 runs..

400 was quite far off may be 17-20 overs were required and this would be too late for declaration 

I was surprised at team's decision but have to respect their decision.....cuz they know better what is good for team.