Firstly, your Grandfather did not steal it, any item of Nazi militaria was considered war booty, this does not fall into the same category as looting or stealing a Nations art treasures or a families personal items all which were covered under military regulations.
At the time this item was liberated it would be worth about as much as toilet paper, which many nazi paper items were used for.
The paper item is more correctly known as a: URKUNDE
which roughly translates as Certificate of Entitlement.
The value today falls into two category's:
1. Historical, some people collect the document for the person it is named for: i.e, Branch of Service, Personal history etc..etc..
2. Signature collectors, here the signature has to be verified that it is Adolf Hitlers real signature and not applied by rubber stamp. Adolf Hitler did not have the time to sign each and every document.
Also it depends on the condition of the document, even with A.H. name if in poor or damaged condition it will seriously effect the value.
Do not effect any repairs with tape or glue if damaged, this drops the value way down!!!
Some of these items have a value as low as 40 Euro to 150+ Euro depending on what I stated above. A picture would be a big help when trying to ask for an evaluation.
P.S. watch out for "nice" people offering to take this worthless document off your hands for a few bucks, they buy low and sell big, the militaria world has its share of vermin dealers out to rip vets families off!!!