The Russian government was unwilling to return the captured documents to Germany, but they wanted to show their goodwill and provide the files for research in this way. I have already heard from some people that the Russians have only released documents which shows the inhumane behavior of the German Wehrmacht. That's absolute nonsense, nothing was censored and nothing is one-sided.
I have found documents that show inhumane behavior, from German and Russian side. The German soldiers was often shocked about the inhumane behavior of Russian commanders, in particular political commissars, against their own troops. There were thousands of Russian deserter who was running to the German side because the political commissars shot their own troops in the back when they couldn't go forward under heavy German fire.
As example on August 2, 1941 the Russians attacked the German 79th Infantry-Division and the half of the Russian attackers was already dead, the attack slowed down. The Germans was surprised and shocked because the last soldiers got shot aimed from the rear with machine guns.
I have a report from August 13, 1941 about a few Russian soldiers who had German flyer about to desert and surrender to the German troops. The political commissars didn't executed only the few soldiers, they let shot the whole unit with around 150 soldiers.
Other documents from German Commanders are about German soldiers who beat Russian prisoner of war. The German officers and soldiers who beat or even shot the Russian POW got a court martial. They came to penal unit, prison and some got capital punishment.
To the rumors of derelict salt mines full of weapons - Of course I can not prove it, but I think it's just rumors. Friendly states of the USSR got these weapons, for example the arsenals in Ukraine were full. From 1990, the last big stocks in Russia were sold.