What's the process through which classified documents are made public, and can some classified documents be destroyed or never made public? I am wondering how the process goes, because I am thinking there might be documents that may jeopardize the country's national security interest even if made public several years after it was made. What's the process like in the United States?
The National Archives has a nice document describing the declassification review process. Yes, I think it is entirely possible that some documents are never declassified. For example Critical Nuclear Weapon Design Information. There are other, less ominous reasons for non-disclosure of government records, for example, personnel records, because personally identifiable information (PII) is protected from disclosure requirements.
A document containing PII isn't necessarily classified, but it is still supposed to be protected against disclosure under the privacy act of 1974. While the president can unilaterally declare something unclassified, he can't ignore the privacy act. So even after declassification, it takes time to redact all the PII.
An interesting example of important classified documents getting lost or destroyed has to do with something codenamed FOGBANK. It seems that FOGBANK was critical to some nuclear weapons designs, and somehow the US forgot how to make FOGBANK.
I'm not sure there was ever a decision to delete the FOGBANK formula, but the information was clearly highly classified and then lost.