What choices has Tom already made when "Contents of the Dead Man's Pocket" opens? Why does he choose to risk his life to retrieve the paper?

Tom Benecke has chosen to stay at home and work on his business proposal rather than to go to the movies with his wife. He knows Clare will be disappointed at having to go to the theater by herself, but this will give him an opportunity to have some privacy for three or four hours to concentrate on writing the final draft of his proposal. Although he lives on the eleventh floor of a Manhattan apartment building and his yellow worksheet containing all his accumulated information has blown out the window, Tom persuades himself it wouldn't be a very serious risk to climb out on the narrow ledge to go after it.

It occurred to him that if this ledge and wall were only a yard above ground — as he knelt at the window staring out, this thought was the final confirmation of his intention — he could move along the ledge indefinitely.

It is only because of the dizzying height that Tom would have any difficulty retrieving his paper. Otherwise, it is just a matter of walking a short distance along a ledge and walking back again. All Tom must do is avoid looking down, although in his situation there is a perverse temptation to do so. Looking down could cause him to have vertigo and fall over backwards into empty space. If the worksheet had simply flown off among the Manhattan skyscrapers, Tom could forget about it, but

he saw that the paper was caught firmly between a projection of the convoluted corner ornament and the ledge. 

How could Tom forget about the worksheet when he wouldn't be able to keep himself from looking out the window every couple minutes to make sure it was still there?

Of all the papers on his desk, why did it have to be this one in particular! On four long Saturday afternoons he had stood in supermarkets counting the people who passed certain displays... From stacks of trade publications... he had copied facts, quotations, and figures onto that sheet. And he had carried it with him to the Public Library on Fifth Avenue, where he'd spent a dozen lunch hours and early evenings adding more. All were needed to support and lend authority to his idea for a new grocery-store display method; without them his idea was a mere opinion. 

Time was of the essence. If Tom procrastinated until Clare came home from the movies, she wouldn't dream of letting him climb out the window onto that ledge. She would think he was utterly insane. It would be better not to tell her he had done such a crazy thing, even after he had actually done it. He wants to be working at the typewriter when she returns home so he can casually ask, "How was the movie?" If he didn't get that worksheet tonight, then tomorrow morning he would have to go to the office. When he returns from work in the early evening and looks out the window, the yellow sheet could be gone forever!
Tom does not believe his life is really in danger until he reaches the paper and must look down for a second to get a grip on it with the tips of his fingers.

He saw, in that instant, the Loew's theater sign, blocks ahead past Fiftieth Street; the miles of traffic signals, all green now; the lights of cars and street lamps; countless neon signs; and the moving black dots of people. And a violent instantaneous explosion of absolute terror roared through him. For a motionless instant he saw himself externally — bent practically double, balanced on this narrow ledge, nearly half his body projecting out above the street far below — and he began to tremble violently, panic flaring through his mind and muscles, and he felt the blood rush from the surface of his skin.

Tom didn't believe he was taking any risk when he got out on that narrow ledge and edged his way to where the paper was stuck. After looking down, though, he feels doomed to die. Tom even imagines his body being found on the sidewalk far below and the police looking through his pockets trying to find out who he was.

All they'd find in his pockets would be the yellow sheet. Contents of the dead man's pockets, he thought, one sheet of paper bearing penciled notations—incomprehensible.