Its actually potentially quite simple, but impossible to answer without the actual text of the pardon. A pardon can be as simple or complex, and as broad or narrow as the President chooses to make it. It could, for instance, cover any offense that may have been committed by the recipient prior to the date of the pardon, in which case the answer is simple -- yes, Snowden would be immune.
Unfortunately, Snowden will never be able to safely return to the US.
He has implied that he possesses such material, in great quantity. He is holding that possession up as deterrent against assassination, although if he received a pardon, presumably he wouldn't need that deterrent anymore?
Besides, I would think a deterrent such as I'm describing would be motivation for russians to assassinate him and make it look like americans did it.
To the contrary, he has adamantly denied possessing any such material. I dont have the source, but in several video interviews he's said to protect himself from being apprehended by foreign intel agencies for a copy of the material, he simply doesnt have it. He's turned it all over to journalists.
He does not possess the material, but he absolutely possesses the encryption keys to the material, and has distributed an encrypted version of a vast trove of material to many different persons, with a deadman switch.
https://www.schneier.com/blog/archives/2013/07/snowdens_dead...
I imagine that a hurdle to pardoning him would be that there is potentially content that would be too egregious not to charge him, which has yet to be released.
If he were pardoned, and that material were released, would he be immune to prosecution? This sounds like a thorny thing to try to figure out, or maybe it's really obvious :)