The most important thing to remember was that the means of production (haha) of the news (the newspaper, radio or later television) was very expensive and therefore quite easily controlled in the pre-Internet era. It's quite expensive to put out a broadsheet or newsletter: you have to acquire some sort of method of mechanical reproduction (typically closely controlled; by the fall of the USSR, the Stasi in East Germany had a record of every typewriter in the country and were able to identify which typewriter had created a given piece of text) and a suspiciously large amount of paper. Similarly, it's nearly impossible to acquire the materials to broadcast a radio station without the government noticing long before you go on air for the first time.

In Nazi Germany, you have the White Rose, which was a group of anti-Nazi students who distributed leaflets and anti-Nazi information. They were all arrested and many were executed.

The Nazis are a special case overall, though, because of course the German state was at war for most of their rule, and thus there was a rather large and well-funded effort to undermine the Nazis in any possible way--including with "documents or intel". The Allied forces broadcast into Germany and Nazi-ruled Europe from the UK and elsewhere--including some stations that pretended to be broadcasting from inside Germany but were in fact coming in from outside. There were also leaflet bombings, in which German-language leaflets were dropped on the home front to undermine morale.

The USSR was slightly different, because there wasn't a hot war happening. The US (as in the actual government) did broadcast Radio Free Europe into the Communist Bloc states, though. I b

Banned texts were often copied as "samizdat," literally translated as self-publishing, which were handwritten or typewritten copies. Readers were expected to make a few (perhaps four) copies of each text they received. There were also hand-reproduced records (on used X-ray film sometimes, very cool) and hand-published newspapers on small hand presses.

I hope this answers your question!