Well, the enigma codes recovered by two Officers of His Majesty's Royal Navy was quite important. I believe that at least one of the two died in the process...

...Then Hollywood came along and re-wrote the story.

[edit]typos, and history check, late October 1942, from U-559, and two sailors died in the process. It seems that several had been recovered before from German weatherships, which had lead to earlier code breakthroughs.

It also appears that some German personnel involve with Enigma failed to believe it had been broken, even when the documentation was released in the 1970s...

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After a substantial amount of badgering from the British press...

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Indeed & I don't know the answer either.

While on the subject of Enigma, the codebooks recovered from U110 by HMS Bulldog on 9th May, 1941 would qualify as documents. These were a vital breakthrough & equally as important as the machine itself. There was already an example of the Enigma machine at Bletchley Park, thanks to the efforts of Polish cipher experts supplying a set of rotors & drawings of the rest of the machine. This had enabled a working example to be built & the Luftwaffe codes were being deciphered during the BoB in 1940.

Another vital breakthrough was when HMS Petard recovered an intact example of the latest 4-rotor Enigma machine used by the German Navy together with the code books from U559. This was the incident on 30th October 1942 when two members of Petard's crew lost their lives. It was thanks to the efforts of young canteen assistant Tommy Brown, who it was later discovered had lied about his age to join the RN, that the vital codebooks were recovered intact. http://www.royalnavalmuseum.org/info_sheets_enigma.htm

PS. I think that Winston Churchill would have regarded these as possibly the most vital documents recovered during WWII. He described the 'U-Boat menace' as his worst nightmare, so serious that if allowed to continue could affect the outcome of the war.

It can be that and be entertaining at the same time.
Though I've never seen it, so I have no room to comment.

I'm going to guess the capture, early on, of the German plans to invade through Belgium.

If I recall, a light plane crash landed on the Belgian side of the lines and before they had a chance to burn the papers, the Belgians captured the officer's briefcase, containing the plans to invade throught the Ardennes.  It was shown to the French/Allied command, but since it didn't fit into the mindset of the Allied commanders, it was discarded as a "plant" and no preparations were made in the dispositions.

Had the Allied command paid heed to the plans, they could have moved troops and armour to head off the attack.  After all, the Allied had more, and better, tanks than the Germans.  The Germans had the better tactics for using tanks.

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That is indeed significant, and it may be argued that it is the most substantial document, but I would suggest since it was ignored, it was rendered ineffective.  The document I am referring to was indeed acted on and was quite significant.

If nobody gets the answer by later today, I'll reveal the document.

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Indeed. WWII covers a long period of time & several different theatres over a vast area in almost every part of the world. With no knowledge of the theatre involved or who considered this document vitally important it's extremely difficult even to hazard a guess.